2 . StreamCorridorFunctionsandDynamicEquilibrium 2.E BiologicalProcesses 2.D ChemicalProcesses 2.C GeomorphicProcesses 2.B Hydrologic andHydraulicProcesses 2.A • • • • • • • • Howishydrologydifferent inurbanstreamcorridors? How fast,howmuch,deep,oftenandwhendoeswaterflow? • What processesaffect orareinvolvedwithstreamflow? • Where doesstreamflowcomefrom? • • • • • • • • • • • • • • • • • • Is thereanimportantrelationshipbetweenastreamanditsfloodplain? What isafloodplain? How dochanneladjustmentsoccur? What shouldachannellooklikeincrosssectionandprofile? What isanequilibriumchannel? Where doessedimentcomefromandhowisittransporteddownstream? How arewaterandsedimentrelated? What factorsaffect thechannelcrosssectionandprofile? What roledofishhaveinstreamcorridorrestoration? What aresomeimportantbiologicalprocessesthatoccurwithinastreamcorridor? of streamcorridors? What arethestructuralfeaturesofaquaticsystemsthatcontributetobiologicaldiversity How doesthestructureofstreamcorridorssupportvariouspopulationsorganisms? What biologicalactivitiesandorganismscanbefoundwithinastreamcorridor? What aretheimportantbiologicalcomponentsofastreamcorridor? water? How dodisturbancesinthestreamcorridoraffect thechemicalcharacteristicsof What arethenaturalchemicalprocessesinastreamcorridorandwatercolumn? stream corridor? How arethechemicalandphysicalparameterscriticaltoaquaticlifeina chemical parameters? What aresomeimportantrelationshipsbetweenphysicalhabitatandkey What arethemajorchemicalconstituentsofwater? (i.e., dynamicequilibrium)? How doesastreamcorridorrespondtoallthenatural forcesactingonit Are thesefunctionsrelated? Is astreamcorridorstable? How aretheseecologicalfunctionsmaintainedovertime? What arethemajorecologicalfunctionsofstreamcorridors? 2 2.A Hydrologic and Hydraulic Processes 2.B Geomorphic Processes 2.C Physical and Chemical Characteristics 2.D Biological Community Characteristics 2.E Functions and Dynamic Equilibrium

hapter 1 provided an overview of stream corridor look and function the way stream corridors and the many per- it does. While Chapter 1 presented still spectives from which they should be images, this chapter provides “film viewed in terms of scale, equilibrium, and footage” to describe the processes, char- space. Each of these views can be seen as acteristics, and functions of stream corri- a “snapshot” of different aspects of a dors through time. stream corridor. Section 2.A: Hydrologic and Hydraulic Chapter 2 presents the stream corridor in Processes motion, providing a basic understanding Understanding how water flows into and of the different processes that make the through stream corridors is critical to restorations. How fast, how much, how deep, how often, and when water flows are important basic questions that must be answered to

Figure 2.1: A stream corridor in motion. Processes, characteris- tics, and functions shape stream corridors and make them look the way they do. make appropriate decisions about nonetheless critical to the functions stream corridor restoration. and processes of stream corridors. Changes in soil or water chemistry Section 2.B: Geomorphic Processes to achieve restoration goals usually This section combines basic hydro- involve managing or altering ele- logic processes with physical or ments in the landscape or corridor. geomorphic functions and charac- teristics. Water flows through Section 2.D: Biological Community but is affected by the kinds Characteristics of soils and alluvial features within The fish, wildlife, plants, and hu- the , in the , and mans that use, live in, or just visit in the uplands. The amount and the stream corridor are key ele- kind of carried by a ments to consider in restoration. stream largely determines its equi- Typical goals are to restore, create, librium characteristics, including enhance, or protect habitat to ben- size, shape, and profile. Successful efit life. It is important to under- stream corridor restoration, stand how water flows, how whether active (requiring direct is transported, and how changes) or passive (management geomorphic features and processes and removal of disturbance fac- evolve; however, a prerequisite to tors), depends on an understanding successful restoration is an under- of how water and sediment are re- standing of the living parts of the lated to channel form and function system and how the physical and and on what processes are involved chemical processes affect the with channel evolution. stream corridor.

Section 2.C: Physical and Chemical Section 2.E: Functions and Characteristics Dynamic Equilibrium The quality of water in the stream The six major functions of stream corridor is normally a primary ob- corridors are: habitat, conduit, jective of restoration, either to im- barrier, filter, source, and sink. prove it to a desired condition, or The integrity of a stream corridor to sustain it. Restoration should ecosystem depends on how well consider the physical and chemical these functions operate. This characteristics that may not be section discusses these functions readily apparent but that are and how they relate to dynamic equilibrium.

2–2 Chapter 2: Stream Corridor Processes, Characteristics, and Functions 2.A Hydrologic and Hydraulic Processes

The hydrologic cycle describes the contin- gions that experience seasonal cycles of uum of the transfer of water from pre- snowfall and snowmelt. cipitation to and ground The type of precipitation that will occur water, to storage and runoff, and to the is generally a factor of humidity and air eventual return to the atmosphere by temperature. Topographic relief and ge- transpiration and evaporation (Figure ographic location relative to large water 2.2). bodies also affect the frequency and Precipitation returns water to the earth’s type of precipitation. Rainstorms occur surface. Although most hydrologic more frequently along coastal and low- processes are described in terms of rain- latitude areas with moderate tempera- fall events (or storm events), snowmelt tures and low relief. Snowfalls occur is also an important source of water, es- more frequently at high elevations and pecially for that originate in high in mid-latitude areas with colder sea- mountain areas and for continental re- sonal temperatures.

cloud formation

rain clouds evaporation

s l

i n

n n m o a

s o

o a e

i i

c t e t

r m o a a t t

s o r

e r i

m g f p

m s precipitation e o o v r r n f f m a r

o t r f

lake storage sur face run off

soil

percolation rock ocean deep percolation

ground w ater

Figure 2.2: The hydrologic cycle. The transfer of water from precipitation to surface water and ground water, to storage and runoff, and eventually back to the atmosphere is an ongoing cycle.

Hydrologic and Hydraulic Processes 2–3 Precipitation can do one of three things intercepted in this manner is determined once it reaches the earth. It can return by the amount of interception storage to the atmosphere; move into the soil; available on the above-ground surfaces. or run off the earth’s surface into a In vegetated areas, storage is a function stream, lake, wetland, or other water of plant type and the form and density body. All three pathways play a role in of leaves, branches, and stems (Table determining how water moves into, 2.1). Factors that affect storage in across, and down the stream forested areas include: corridor. Leaf shape. Conifer needles hold This section is divided into two subsec- water more efficiently than leaves. tions. The first subsection focuses on On leaf surfaces droplets run togeth- hydrologic and hydraulic processes in er and roll off. Needles, however, the lateral dimension, namely, the keep droplets separated. movement of water from the land into the channel. The second subsection Leaf texture. Rough leaves store more concentrates on water as it moves in the water than smooth leaves. longitudinal dimension, specifically as Time of year. Leafless periods provide streamflow in the channel. less interception potential in the canopy than growing periods; howev- Hydrologic and Hydraulic er, more storage sites are created by Processes Across the Stream leaf litter during this time. Corridor Vertical and horizontal density. The Key points in the hydrologic cycle serve more layers of vegetation that precip- as organizational headings in this sub- itation must penetrate, the less likely section: it is to reach the soil. Interception, transpiration, and Age of the plant community. Some evapotranspiration. vegetative stands become more dense Infiltration, soil moisture, and with age; others become less dense. ground water. The intensity, duration, and frequency Runoff. of precipitation also affect levels of in- terception. Interception, Transpiration, and Evapotranspiration Figure 2.3 shows some of the pathways rainfall can take in a forest. Rainfall at More than two-thirds of the precipita- Table 2.1: Percentage of precipitation inter- tion falling over the United States evap- cepted for various vegetation types. orates to the atmosphere rather than Source: Dunne and Leopold 1978. being discharged as streamflow to the Vegetative Type % Precipitation Intercepted oceans. This “short-circuiting” of the Forests hydrologic cycle occurs because of the Deciduous 13 two processes, interception and transpi- ration. Coniferous 28 Crops Interception Alfalfa 36 A portion of precipitation never reaches Corn 16 the ground because it is intercepted by Oats 7 vegetation and other natural and con- Grasses 10–20 structed surfaces. The amount of water

2–4 Chapter 2: Stream Corridor Processes, Characteristics, and Functions the beginning of a storm initially fills precipitation interception storage sites in the canopy. canopy As the storm continues, water held in interception these storage sites is displaced. The dis- and evaporation placed water drops to the next lower layer of branches and limbs and fills storage sites there. This process is re- peated until displaced water reaches the lowest layer, the leaf litter. At this point, water displaced off the leaf litter either infiltrates the soil or moves downslope as . Antecedent conditions, such as mois- throughfall ture still held in place from previous litter storms, affect the ability to intercept interception stemflow and store additional water. Evaporation and evaporation will eventually remove water residing understory in interception sites. How fast this throughfall process occurs depends on climatic throughfall conditions that affect the evaporation rate. Interception is usually insignificant in litter areas with little or no vegetation. Bare net rainfall entering soil or rock has some small imperme- mineral soil the soil able depressions that function as inter- ception storage sites, but typically most Figure 2.3: Typical pathways for forest rainfall. of the precipitation either infiltrates the A portion of precipitation never reaches the soil or moves downslope as surface ground because it is intercepted by vegetation runoff. In areas of frozen soil, intercep- and other surfaces. tion storage sites are typically filled with frozen water. Consequently, addi- water originates from water taken in by tional rainfall is rapidly transformed roots. into surface runoff. Transpiration from vegetation and evap- Interception can be significant in large oration from interception sites and urban areas. Although urban drainage open water surfaces, such as ponds and systems are designed to quickly move lakes, are not the only sources of water storm water off impervious surfaces, the returned to the atmosphere. Soil mois- urban landscape is rich with storage ture also is subject to evaporation. sites. These include flat rooftops, park- Evaporation of soil moisture is, how- ing lots, potholes, cracks, and other ever, a much slower process due to cap- rough surfaces that can intercept and illary and osmotic forces that keep the hold water for eventual evaporation. moisture in the soil and the fact that Transpiration and Evapotranspiration vapor must diffuse upward through soil Transpiration is the diffusion of water pores to reach surface air at a lower vapor from plant leaves to the atmos- vapor pressure. phere. Unlike intercepted water, which Because it is virtually impossible to sep- originates from precipitation, transpired arate water loss due to transpiration

Hydrologic and Hydraulic Processes 2–5 The net rate of movement is proportional to the difference in vapor pressure between the water Water is subject to evaporation whenever it is surface and the atmosphere above that surface. exposed to the atmosphere. Basically this process Once the pressure is equalized, no more evapora- involves: tion can occur until new air, capable of holding ■ The change of state of water from liquid to more water vapor, displaces the old saturated air. vapor Evaporation rates therefore vary according to lati- ■ The net transfer of this vapor to the atmosphere tude, season, time of day, cloudiness, and wind The process begins when some molecules in the energy. Mean annual lake evaporation in the liquid state attain sufficient kinetic energy (primari- United States, for example, varies from 20 inches ly from solar energy) to overcome the forces of in Maine and Washington to about 86 inches in surface tension and move into the atmosphere. the desert Southwest (Figure 2.4). This movement creates a vapor pressure in the atmosphere.

<20 inches 20–30 inches 30–40 inches 40–50 inches 50–60 inches 60–70 inches 70–80 inches >80 inches

Figure 2.4: Mean annual lake evaporation for the period 1946–1955. Source: Dunne and Leopold (1978) modified from Kohler et al. (1959).

2–6 Chapter 2: Stream Corridor Processes, Characteristics, and Functions from water loss due to evaporation, the rain two processes are commonly combined and labeled evapotranspiration. Evapo- transpiration can dominate the water balance and can control soil moisture content, ground water recharge, and wetted streamflow. grains The following concepts are important when describing evapotranspiration:

If soil moisture conditions are limit- gravitational ing, the actual rate of evapotranspira-

tion is below its potential rate. force dry When vegetation loses water to the grains atmosphere at a rate unlimited by the supply of water replenishing the roots, its actual rate of evapotranspi- ration is equal to its potential rate of evapotranspiration. rain The amount of precipitation in a region drives both processes, however. Soil types and rooting characteristics also play important roles in determining the actual rate of evapotranspiration. wetted grains Infiltration, Soil Moisture, and Ground Water capillary

Precipitation that is not intercepted or force flows as surface runoff moves into the soil. Once there, it can be stored in the upper layer or move downward through dry grains the soil profile until it reaches an area completely saturated by water called the phreatic zone. Infiltration Close examination of the soil surface re- dry capillary grains

veals millions of particles of sand, silt, force and clay separated by channels of differ- ent sizes (Figure 2.5). These macropores include cracks, “pipes” left by decayed roots and wormholes, and pore spaces between lumps and particles of soil. wetted Water is drawn into the pores by gravity grains and capillary action. Gravity is the dominant force for water moving into the largest openings, such as worm or Figure 2.5: Soil profile. Water is drawn into the root holes. Capillary action is the domi- pores in soil by gravity and capillary action.

Hydrologic and Hydraulic Processes 2–7 rainfall rainfall .75 inches/hr 1.5 inches/hr

runoff 0.5 inches/hr

infiltration infiltration .75 inches/hr 1 inch/hr

A. Infiltration Rate = B. Runoff Rate = rainfall rate, which is less than rainfall rate minus infiltration capacity infiltration capacity

Figure 2.6: Infiltration and runoff. Surface runoff occurs when rainfall intensity exceeds infiltration capacity.

nant force for water moving into soils pacity, the excess water either is de- with very fine pores. tained in small depressions on the soil The size and density of these pore surface or travels downslope as surface openings determine the water’s rate of runoff (Figure 2.6). entry into the soil. Porosity is the term The following factors are important in used to describe the percentage of the determining a soil’s infiltration rate: total soil volume taken up by spaces be- Ease of entry through the soil surface. tween soil particles. When all those spaces are filled with water, the soil is Storage capacity within the soil. said to be saturated. Transmission rate through the soil. Soil characteristics such as texture and Areas with natural vegetative cover and tilth (looseness) are key factors in deter- leaf litter usually have high infiltration mining porosity. Coarse-textured, sandy rates. These features protect the surface soils and soils with loose aggregates soil pore spaces from being plugged by held together by organic matter or small fine soil particles created by raindrop amounts of clay have large pores and, splash. They also provide habitat for thus, high porosity. Soils that are tightly worms and other burrowing organisms packed or clayey have low porosity. and provide organic matter that helps Infiltration is the term used to describe bind fine soil particles together. Both of the movement of water into soil pores. these processes increase porosity and The infiltration rate is the amount of the infiltration rate. water that soaks into soil over a given The rate of infiltration is not constant length of time. The maximum rate that throughout the duration of a storm. water infiltrates a soil is known as the The rate is usually high at the begin- soil’s infiltration capacity. ning of a storm but declines rapidly as If rainfall intensity is less than infiltra- gravity-fed storage capacity is filled. tion capacity, water infiltrates the soil at A slower, but stabilized, rate of infiltra- a rate equal to the rate of rainfall. If the tion is reached typically 1 or 2 hours rainfall rate exceeds the infiltration ca- into a storm. Several factors are in-

2–8 Chapter 2: Stream Corridor Processes, Characteristics, and Functions volved in this stabilization process, 0.60

including the following: porosity Raindrops breaking up soil aggregates 0.50 and producing finer material, which then blocks pore openings on the sur-

face and reduces the ease of entry. 0.40 Water filling fine pore spaces and field unfilled capacity reducing storage capacity. pore space 0.30 Wetted clay particles swelling and wilting effectively reducing the diameter of point 0.20 pore spaces, which, in turn, reduces Proportion by Volume transmission rates. Soils gradually drain or dry following a 0.10 storm. However, if another storm occurs clay before the drying process is completed, loam heavy clay loam there is less storage space for new water. 0 fine clay loam sandy loam Therefore, antecedent moisture condi- light clay loam sandy loam tions are important when analyzing silt loam fine sand available storage. sand Soil Moisture Figure 2.7: Water-holding properties of various After a storm passes, water drains out of soils. Water-holding properties vary by texture. upper soils due to gravity. The soil re- For a fine sandy loam the approximate differ- mains moist, however, because some ence between porosity, 0.45, and field capacity, amount of water remains tightly held in 0.20, is 0.25, meaning that the unfilled pore space is 0.25 times the soil volume. The differ- fine pores and around particles by sur- ence between field capacity and wilting point is face tension. This condition, called field a measure of unfilled pore space. capacity, varies with soil texture. Like Source: Dunne and Leopold 1978. porosity, it is expressed as a proportion by volume. pore water. The moisture content of the The difference between porosity and soil at this point, which varies depend- field capacity is a measure of unfilled ing on soil characteristics, is called the pore space (Figure 2.7). Field capacity permanent wilting point because plants is an approximate number, however, be- can no longer withdraw water from the cause gravitation drainage continues in soil at a rate high enough to keep up moist soil at a slow rate. with the demands of transpiration, caus- ing the plants to wilt. Soil moisture is most important in the context of evapotranspiration. Terrestrial Deep percolation is the amount of water plants depend on water stored in soil. that passes below the root zone of As their roots extract water from pro- crops, less any upward movement of gressively finer pores, the moisture con- water from below the root zone (Jensen tent in the soil may fall below the field et al. 1990). capacity. If soil moisture is not replen- Ground Water ished, the roots eventually reach a point where they cannot create enough suc- The size and quantity of pore openings tion to extract the tightly held interstitial also determines the movement of water within the soil profile. Gravity causes

Hydrologic and Hydraulic Processes 2–9 water to move vertically downward. aeration. It contains air and microbial This movement occurs easily through respiratory gases, capillary water, and larger pores. As pores reduce in size due water moving downward by gravity to to swelling of clay particles or filling of the phreatic zone. Pellicular water is the pores, there is a greater resistance to film of ground water that adheres to in- flow. Capillary forces eventually take dividual particles above the ground over and cause water to move in any water table. This water is held above the direction. capillary fringe by molecular attraction. Water will continue to move downward If the phreatic zone provides a consis- until it reaches an area completely satu- tent supply of water to wells, it is rated with water, the phreatic zone or known as an . Good zone of saturation (Figure 2.8). The top usually have a large lateral and vertical of the phreatic zone defines the ground extent relative to the amount of water water table or phreatic surface. Just withdrawn from wells and high poros- above the ground water table is an area ity, which allows water to drain easily. called the capillary fringe, so named be- The opposite of an aquifer is an cause the pores in this area are filled aquitard or confining bed. Aquitards or with water held by capillary forces. confining beds are relatively thin sedi- In soils with tiny pores, such as clay or ment or rock layers that have low per- silt, the capillary forces are strong. Con- meability. Vertical water movement sequently, the capillary fringe can ex- through an aquitard is severely re- tend a large distance upward from the stricted. If an aquifer has no confining water table. In sandstone or soils with layer overlying it, it is known as an large pores, the capillary forces are weak unconfined aquifer. A confined aquifer is and the fringe narrow. one confined by an aquitard. Between the capillary fringe and the soil The complexity and diversity of aquifers surface is the vadose zone, or the zone of and aquitards result in a multitude of

Figure 2.8: Ground water related fea- potentimetric flowing perched water water table land tures and terminolo- surface artesian table and aquifer well surface gy. Ground water well seep losing elevation along the gaining stream stream corridor can stream capillary fringe vary significantly over confining vadose zone bed short distances, water depending on subsur- table face characteristics. Source: USGS Water water table Supply Paper #1988, 972, Definitions of unconfined aquifer zone of Selected Ground Water saturation Terms. confining bed

ground water (phreatic water)

aquitard confined aquifer

bedrock

2–10 Chapter 2: Stream Corridor Processes, Characteristics, and Functions underground scenarios. For example, channel can function either as a perched ground water occurs when a shal- recharge area (influent or “losing” low aquitard of limited size prevents stream) or a area (effluent water from moving down to the or “gaining” stream). phreatic zone. Water collects above the aquitard and forms a “mini-phreatic Runoff zone.” In many cases, perched ground When the rate of rainfall or snowmelt water appears only during a storm or exceeds infiltration capacity, excess during the wet season. Wells tapping water collects on the soil surface and perched ground water may experience a travels downslope as runoff. Factors shortage of water during the dry season. that affect runoff processes include cli- Perched aquifers can, however, be im- mate, geology, topography, soil charac- portant local sources of ground water. teristics, and vegetation. Average annual Artesian wells are developed in con- runoff in the contiguous United States fined aquifers. Because the hydrostatic ranges from less than 1 inch to more pressure in confined aquifers is greater than 20 inches (Figure 2.9). than atmospheric pressure, water levels Three basic types of runoff are intro- in artesian wells rise to a level where at- duced in this subsection (Figure 2.10): mospheric pressure equals hydrostatic pressure. If this elevation is above the Overland flow ground surface, water can flow freely Subsurface flow out of the well. Saturated overland flow Water also will flow freely where the Each of these runoff types can occur in- ground surface intersects a confined dividually or in some combination in aquifer. The piezometric surface is the the same locale. level to which water would rise in wells tapped into confined aquifers if the Overland Flow wells extended indefinitely above the When the rate of precipitation exceeds ground surface. Phreatic wells draw the rate of infiltration, water collects on water from below the phreatic zone in the soil surface in small depressions unconfined aquifers. The water level in (Figure 2.11). The water stored in these a phreatic well is the same as the spaces is called depression storage. It ground water table. eventually is returned to the atmos- Practitioners of stream corridor restora- phere through evaporation or infiltrates tion should be concerned with locations the soil surface. where ground water and surface water After depression storage spaces are filled, are exchanged. Areas that freely allow excess water begins to move downslope movement of water to the phreatic zone as overland flow, either as a shallow are called recharge areas. Areas where the sheet of water or as a series of small water table meets the soil surface or rivulets or . Horton (1933) was the where stream and ground water emerge first to describe this process in the liter- are called springs or seeps. ature. The term Horton overland flow or The volume of ground water and the Hortonian flow is commonly used. elevation of the water table fluctuate The sheet of water increases in depth according to ground water recharge and velocity as it moves downhill. As it and discharge. Because of the fluctua- travels, some of the overland flow is tion of water table elevation, a stream trapped on the hillside and is called sur-

Hydrologic and Hydraulic Processes 2–11 <1 inch Figure 2.9: Average 1–10 inches annual runoff in the 10–20 inches contiguous United >20 inches States. Average annual runoff varies with regions. Source: USGS 1986.

face detention. Unlike depression stor- tion, the water table before a rainstorm age, which evaporates to the atmos- has a parabolic surface that slopes to- phere or enters the soil, surface ward a stream. Water moves downward detention is only temporarily detained and along this slope and into the from its journey downslope. It eventu- stream channel. This portion of the ally runs off into the stream and is still flow is the . The soil below the considered part of the total volume of water table is, of course, saturated. As- overland flow. suming the hill slope has uniform soil Overland flow typically occurs in urban characteristics, the moisture content of and suburban settings with paved and surface soils diminishes with distance impermeable surfaces. Paved areas and from the stream. soils that have been exposed and com- During a storm, the soil nearest the pacted by heavy equipment or vehicles stream has two important attributes as are also prime settings for overland compared to soil upslope—a higher flow. It is also common in areas of thin moisture content and a shorter distance soils with sparse vegetative cover such to the water table. These attributes cause as in mountainous terrain of arid or the water table to rise more rapidly in semiarid regions. response to rainwater infiltration and causes the water table to steepen. Thus a Subsurface Flow new, storm-generated ground water Once in the soil, water moves in re- component is added to baseflow. This sponse to differences in hydraulic head new component, called subsurface flow, (the potential for flow due to the gradi- mixes with baseflow and increases ent of hydrostatic pressure at different ground water discharge to the channel. elevations). Given a simplified situa-

2–12 Chapter 2: Stream Corridor Processes, Characteristics, and Functions precipitation

litt er la y

e precipitation r

H o Figure 2.10: Flow r to paths of water over n o a surface. The por- ve rla tion of precipitation sh nd saturated a fl llo ow overland that runs off or g w r flow o s infiltrates to the ub u su

n r fac ground water table d e f low depends on the soil’s w permeability rate; a t water er surface roughness; fl table ow and the amount, duration, and intensi- ty of precipitation.

In some situations, infiltrated storm pands further up the hillside. Because water does not reach the phreatic zone quick return flow and subsurface flow because of the presence of an aquitard. are so closely linked to overland flow, In this case, subsurface flow does not they are normally considered part of mix with baseflow, but also discharges the overall runoff of surface water. water into the channel. The net result, whether mixed or not, is increased Hydrologic and Hydraulic channel flow. Processes Along the Stream Corridor Saturated Overland Flow Water flowing in streams is the collection If the storm described above continues, of direct precipitation and water that the slope of the water table surface can has moved laterally from the land into continue to steepen near the stream. the channel. The amount and timing of Eventually, it can steepen to the point this lateral movement directly influences that the water table rises above the Figure 2.11: Overland flow and depression channel elevation. Additionally, ground storage. Overland moves downslope as an water can break out of the soil and irregular sheet. travel to the stream as overland flow. Source: Dunne and Leopold 1978. This type of runoff is termed quick return flow. surface detention depth and The soil below the ground water break- velocity of overland flow out is, of course, saturated. Conse- increase quently, the maximum infiltration rate downslope is reached, and all of the rain falling on it flows downslope as overland runoff. The combination of this direct precipitation and quick return flow is stream depression storage channel called saturated overland flow. As the (depth of depressions storm progresses, the saturated area ex- greatly exaggerated)

Hydrologic and Hydraulic Processes 2–13 the amount and timing of streamflow, information is usually presented in a which in turn influences ecological probability format. Two formats are es-

FAST functions in the stream corridor. pecially useful for planning and design- FORWARD ing stream corridor restoration: Flow Analysis Flow duration, the probability a given Flows range from no flow to flows streamflow was equaled or exceeded in a variety of time scales. On a broad over a period of time. Preview Chap- scale, historical climate records reveal Flow frequency, the probability a ter 7, Section occasional persistent periods of wet and given streamflow will be exceeded A for more de- dry years. Many rivers in the United (or not exceeded) in a year. tailed informa- States, for example, experienced a de- (Sometimes this concept is modified tion about cline in flows during the “dust bowl” and expressed as the average number flow duration decade in the 1930s. Another similar de- and frequency. of years between exceeding [or not cline in flows nationwide occurred in exceeding] a given flow.) the 1950s. Unfortunately, the length of record regarding wet and dry years is Figure 2.12 presents an example of a short (in geologic time), making it is flow frequency expressed as a series of difficult to predict broad-scale persis- probability curves. The graph displays tence of wet or dry years. months on the x-axis and a range of mean monthly discharges on the y-axis. Seasonal variations of streamflow are The curves indicate the probability that more predictable, though somewhat the mean monthly discharge will be complicated by persistence factors. Be- less than the value indicated by the cause design work requires using histor- curve. For example, on about January 1, ical information (period of record) as a there is a 90 percent chance that the basis for designing for the future, flow

15000

90%

10000

75%

5000 50%

Mean Monthly Discharge (cfs) 25%

10% 0 Oct. Nov. Dec. Jan. Feb. Mar. April May June July Aug. Sept. Month

Figure 2.12: An example of monthly probability curves. Monthly probability that the mean monthly discharge will be less than the values indicated. Yakima near Parker, Washington. (Data from U.S. Army Corps of Engineers.) Source: Dunne and Leopold 1978.

2–14 Chapter 2: Stream Corridor Processes, Characteristics, and Functions discharge will be less than 9,000 cfs populations of a single species in sev- and a 50 percent chance it will be less eral locations. than 2,000 cfs. In general, completion of the life cycle Ecological Impacts of Flow of many riverine species requires an array of different habitat types whose The variability of streamflow is a pri- temporal availability is determined mary influence on the biotic and abiotic by the flow regime. Adaptation to this processes that determine the structure environmental dynamism allows river- and dynamics of stream ecosystems ine species to persist during periods (Covich 1993). High flows are impor- of droughts and that destroy tant not only in terms of sediment and recreate habitat elements (Poff transport, but also in terms of recon- et al. 1997). necting floodplain wetlands to the channel. This relationship is important because floodplain wetlands provide spawning and nursery habitat for fish and, later in the year, foraging habitat for waterfowl. Low flows, especially in large rivers, create conditions that allow fauna to disperse, thus maintaining

2.B Geomorphic Processes

Geomorphology is the study of surface Sediment , settling of erod- forms of the earth and the processes ed soil particles to the bottom of a that developed those forms. The hydro- water body or left behind as water logic processes discussed in the previ- leaves. Sediment deposition can be ous section drive the geomorphic transitory, as in a stream channel processes described in this section. In from one storm to another, or more turn, the geomorphic processes are the or less permanent, as in a larger primary mechanisms for forming the reservoir. drainage patterns, channel, floodplain, Since geomorphic processes are so terraces, and other watershed and closely related to the movement of stream corridor features discussed in water, this section is organized into Chapter 1. subsections that mirror the hydrologic Three primary geomorphic processes processes of surface storm water runoff are involved with flowing water, as fol- and streamflow: lows: Geomorphic Processes Across the , the detachment of soil parti- Stream Corridor cles. Geomorphic Processes Along the , the movement of Stream Corridor eroded soil particles in flowing water.

Geomorphic Processes 2–15 Geomorphic Processes Across No matter the size, all particles in the the Stream Corridor channel are subject to being trans- ported downslope or downstream. The occurrence, magnitude, and distrib- The size of the largest particle a stream ution of erosion processes in water- can move under a given set of hy- sheds affect the yield of sediment and draulic conditions is referred to as associated water quality contaminants stream competence. Often, only very to the stream corridor. high flows are competent to move the Soil erosion can occur gradually over largest particles. a long period, or it can be cyclic or Closely related to stream competence is episodic, accelerating during certain the concept of tractive stress, which cre- seasons or during certain rainstorm ates lift and drag forces at the stream events (Figure 2.13). Soil erosion can boundaries along the bed and banks. be caused by human actions or by nat- Tractive stress, also known as shear ural processes. Erosion is not a simple stress, varies as a function of flow depth process because soil conditions are con- and slope. Assuming constant density, tinually changing with temperature, shape, and surface roughness, the larger moisture content, growth stage and the particle, the greater the amount of amount of vegetation, and the human tractive stress needed to dislodge it and manipulation of the soil for develop- move it downstream. ment or crop production. Tables 2.2 and 2.3 show the basic processes that The energy that sets sediment particles influence soil erosion and the different into motion is derived from the effect types of erosion found within the water- of faster water flowing past slower shed. water. This velocity gradient happens because the water in the main body of Geomorphic Processes Along flow moves faster than water flowing at the Stream Corridor the boundaries. This is because bound- The channel, floodplain, terraces, and other features in the stream corridor are formed primarily through the erosion, transport, and deposition of sediment by streamflow. This subsection de- scribes the processes involved with transporting sediment loads down- stream and how the channel and floodplain adjust and evolve through time.

Sediment Transport Sediment particles found in the stream channel and floodplain can be catego- rized according to size. A boulder is the largest particle and clay is the smallest particle. Particle density depends on the size and composition of the particle Figure 2.13: Raindrop impact. One of many (i.e., the specific gravity of the mineral types of erosion. content of the particle).

2–16 Chapter 2: Stream Corridor Processes, Characteristics, and Functions aries are rough and create friction as Table 2.2: Erosion processes. flow moves over them which, in turn, slows flow. Agent Process Raindrop impact Sheet, interill The momentum of the faster water is Surface water runoff Sheet, interill, , ephemeral , classic gully transmitted to the slower boundary water. In doing so, the faster water Channelized flow Rill, ephemeral gully, classic gully, wind, streambank tends to roll up the slower water in a Gravity Classic gully, streambank, landslide, mass wasting spiral motion. It is this shearing mo- Wind Wind tion, or shear stress, that also moves Ice Streambank, lake shore bed particles in a rolling motion down- Chemical reactions Solution, dispersion stream. Table 2.3: Erosion types vs. physical processes. Particle movement on the channel bot- tom begins as a sliding or rolling mo- Erosion/Physical Process tion, which transports particles along Erosion Type Sheet Concentrated Mass Combination the streambed in the direction of flow Flow Wasting (Figure 2.14). Some particles also may Sheet and rill x x move above the bed surface by , Interill x a skipping motion that occurs when Rill x x one particle collides with another parti- Wind x x cle, causing it to bounce upward and Ephemeral gully x then fall back toward the bed. Classic gully x x These rolling, sliding, and skipping mo- Floodplain scour x tions result in frequent contact of the Roadside x moving particles with the streambed Streambank x x and characterize the set of moving par- Streambed x ticles known as . The weight of Landslide x these particles relative to flow velocity causes them essentially to remain in Wave/shoreline x contact with, and to be supported by, Urban, construction x the streambed as they move down- Surface mine x stream. Ice gouging x

Direction of shear due to Tendency of Suggested motion of a decrease of velocity to roll grain thrown up into velocity an exposed Diagram of turbulent eddies in the toward bed. grain. saltating grains. flow.

Figure 2.14: Action of water on particles near the streambed. Processes that transport bed load sediments are a function of flow velocities, particle size, and principles of hydrodynamics. Source: Water in Environmental Planning by Dunne and Leopold © 1978 by W.H. Freeman and Company. Used with permission.

Geomorphic Processes 2–17 Part of the may be col- loidal clays, which can remain in sus- pension for very long time periods, depending on the type of clay and One way to differentiate the sediment load of a stream water chemistry. is to characterize it based on the immediate source of Sediment Transport Terminology the sediment in transport. The total sediment load in a Sediment transport terminology can stream, at any given time and location, is divided into sometimes be confusing. Because of two parts— and bed-material load. The prima- this confusion, it is important to define ry source of wash load is the watershed, including sheet some of the more frequently used and rill erosion, gully erosion, and upstream streambank terms. erosion. The source of is primarily the streambed itself, but includes other sources in the water- Sediment load, the quantity of sedi- shed. ment that is carried past any cross section of a stream in a specified Wash load is composed of the finest sediment particles period of time, usually a day or a in transport. Turbulence holds the wash load in suspen- year. Sediment discharge, the mass sion. The concentration of wash load in suspension is or volume of sediment passing a essentially independent of hydraulic conditions in the stream cross section in a unit of stream and therefore cannot be calculated using mea- time. Typical units for sediment load sured or estimated hydraulic parameters such as velocity are tons, while sediment discharge or discharge. Wash load concentration is normally a units are tons per day. function of supply; i.e., the stream can carry as much wash load as the watershed and banks can deliver (for Bed-material load, part of the total sediment concentrations below approximately 3000 sediment discharge that is composed parts per million). of sediment particles that are the Bed-material load is composed of the sediment of size same size as streambed sediment. classes found in the streambed. Bed-material load moves Wash load, part of the total sediment along the streambed by rolling, sliding, or jumping, and load that is comprised of particle may be periodically entrained into the flow by turbu- sizes finer than those found in the lence, where it becomes a portion of the suspended streambed. load. Bed-material load is hydraulically controlled and Bed load, portion of the total sedi- can be computed using sediment transport equations ment load that moves on or near the discussed in Chapter 8. streambed by saltation, rolling, or sliding in the bed layer.

Suspended bed material load, portion of the bed material load that is trans- Finer-grained particles are more easily ported in suspension in the water carried into suspension by turbulent ed- column. The suspended bed material dies. These particles are transported load and the bed load comprise the within the water column and are there- total bed material load. fore called the suspended load. Although Suspended sediment discharge (or sus- there may be continuous exchange of pended load), portion of the total sed- sediment between the bed load and iment load that is transported in sus- suspended load of the river, as long as pension by turbulent fluctuations sufficient turbulence is present. within the body of flowing water.

2–18 Chapter 2: Stream Corridor Processes, Characteristics, and Functions Measured load, portion of the total Stream Power sediment load that is obtained by the One of the principal geomorphic tasks sampler in the sampling zone. of a stream is to transport particles out Unmeasured load, portion of the total of the watershed (Figure 2.15). In this sediment load that passes beneath manner, the stream functions as a trans- the sampler, both in suspension and porting “machine;” and, as a machine, on the bed. With typical suspended its rate of doing work can be calculated sediment samplers this is the lower as the product of available power multi- 0.3 to 0.4 feet of the vertical. plied by efficiency. The above terms can be combined in Stream powercan be calculated as: anumber of ways to give the total ϕ= γQ S sediment load in a stream (Table 2.4). However, it is important not to com- Where: bine terms that are not compatible. ϕ = Stream power (foot-lbs/second- Forexample, the suspended load and foot) the bed material load are not compli- mentary terms because the suspended γ= Specific weight of water (lbs/ft3) load may include a portion of the bed Q = Discharge (ft3/second) material load, depending on the energy available for transport. The total sedi- S = Slope (feet/feet) ment load is correctly defined by the Sediment transport rates are directly re- combination of the following terms: lated to stream power; i.e., slope and discharge. Baseflow that follows the Total Sediment Load = highly sinuous (the line that Bed Material Load + Wash Load marks the deepest points along the or stream channel) in a meandering Bed Load + Suspended Load stream generates little stream power; or therefore, the stream’s ability to move sediment, sediment-transport capacity,is Measured Load + Unmeasured Load limited. At greater depths, the flow fol- Sediment transport rates can be com- lows a straighter course, which increases puted using various equations or mod- slope, causing increased sediment trans- els. These are discussed in the Stream port rates. The stream builds its cross Channel Restorationsection of Chapter8. section to obtain depths of flow and channel slopes that generate the sedi- Table 2.4: Sediment load terms. ment-transport capacity needed to maintain the stream channel. Classification System Runoff can vary from a watershed, ei- Based on Based on Mechanism Particle Size ther due to natural causes or land use of Transport practices. These variations may change Wash load Suspended Wash load the size distribution of sediments deliv- load ered to the stream from the watershed by preferentially moving particular par- Suspended Bed-material bed-material load ticle sizes into the stream. It is not un- load common to find a layer of sand on top of a cobble layer. This often happens otal sediment load T when accelerated erosion of sandy soils Bed load Bed load

Geomorphic Processes 2–19 First Order Stream Second to Fourth Order Stream Fifth to Tenth Order Stream

typical flow rate

average particle size on stream bottom

Figure 2.15: Particle transport. A stream’s total sediment load is the total of all sediment particles moving past a defined cross section over a specified time period. Transport rates vary according to the mechanism of transport.

occurs in a watershed and the increased is, the distribution of particle sizes in load of sand exceeds the transport ca- each section of the stream remains in pacity of the stream during events that equilibrium (i.e., new particles de- move the sand into the channel. posited are the same size and shape as particles displaced by tractive stress). Stream and Floodplain Stability Yang (1971) adapted the basic theories A question that normally arises when described by Leopold to explain the considering any ac- longitudinal profile of rivers, the forma- tion is “Is it stable now and will it be tion of stream networks, , and stable after changes are made?” The an- pools, and river meandering. All these swer may be likened to asking an opin- river characteristics and sediment trans- ion on a movie based on only a few port are closely related. Yang (1971) de- frames from the reel. Although we often veloped the theory of average stream view streams based on a limited refer- fall and the theory of least rate of en- ence with respect to time, it is impor- ergy expenditure, based on the entropy tant that we consider the long-term concept. These theories state that during changes and trends in channel cross the evolution toward an equilibrium section, longitudinal profile, and plan- condition, a natural stream chooses its form morphology to characterize chan- course of flow in such a manner that nel stability. the rate of potential energy expenditure Achieving channel stability requires that per unit mass of flow along its course is the average tractive stress maintains a a minimum. stable streambed and streambanks. That

2–20 Chapter 2: Stream Corridor Processes, Characteristics, and Functions Corridor Adjustments ment load (Q ) and median particle s size on the streambed (D ): Stream channels and their 50 Q • D ∼ Q • S are constantly adjusting to the water s 50 w and sediment supplied by the water- Lane’s relationship suggests that a chan- shed. Successful restoration of degraded nel will be maintained in dynamic streams requires an understanding of equilibrium when changes in sediment watershed history, including both nat- load and bed-material size are balanced ural events and land use practices, and by changes in streamflow or channel the adjustment processes active in chan- gradient. A change in one of these vari- nel evolution. ables causes changes in one or more of Channel response to changes in water the other variables such that dynamic and sediment yield may occur at differ- equilibrium is reestablished. FAST ing times and locations, requiring vari- Additional qualitative relationships FORWARD ous levels of energy expenditure. Daily have been proposed for interpreting be- changes in streamflow and sediment havior of alluvial channels. Schumm load result in frequent adjustment of (1977) suggested that width (b), depth bedforms and roughness in many (d), and wavelength (L) are Preview Section streams with movable beds. Streams directly proportional, and that channel E for a further also adjust periodically to extreme high- gradient (S) is inversely proportional to discussion of and low-flow events, as floods not only dynamic equi- streamflow (Qw) in an alluvial channel: remove vegetation but create and in- librium. crease vegetative potential along the ______stream corridor (e.g., low flow periods Q ∼ b, d, L w S allow vegetation incursion into the channel). Schumm (1977) also suggested that Similar levels of adjustment also may width (b), meander wavelength (L), be brought about by changes in land and channel gradient (S) are directly use in the stream corridor and the up- proportional, and that depth (d) and land watershed. Similarly, long-term sinuosity (P) are inversely proportional

changes in runoff or sediment yield to sediment discharge (Qs) in alluvial from natural causes, such as climate streams: change, wildfire, etc., or human causes, ______such as cultivation, overgrazing, or Q ∼ b, L, S rural-to-urban conversions, may lead to s d, P long-term adjustments in channel cross section and planform that are fre- The above two equations may be rewrit- quently described as channel evolution. ten to predict direction of change in channel characteristics, given an in- Stream channel response to changes in crease or decrease in streamflow or sedi- flow and sediment load have been de- ment discharge: scribed qualitatively in a number of Q + ∼ b+, d+, L+, S– studies (e.g., Lane 1955, Schumm w 1977). As discussed in Chapter 1, one Q – ∼ b–, d–, L–, S+ of the earliest relationships proposed w for explaining stream behavior was sug- Q + ∼ b+, d–, L+, S+, P– gested by Lane (1955), who related s

mean annual streamflow (Qw) and channel slope (S) to bed-material sedi- Q – ∼ b–, d+, L–, S–, P+ s

Geomorphic Processes 2–21 Combining the four equations above energy dissipation rate (Yang and Song yields additional predictive relation- 1979), the following equation must be ships for concurrent increases or de- satisfied: creases in streamflow and/or sediment discharge: dP dS dQ ___ = γQ ___ + S ___ =0 + + ∼ + +/– + +/– – Qw Qs b , d , L , S , P dx dx dx

– – ∼ – +/– – +/– + Qw Qs b , d , L , S , P Where: P= QS= Stream power Q +Q – ∼b+/–, d+, L+/–, S–, P+ w s x =Longitudinal distance - + ∼ +/– – +/– + – Qw Qs b , d , L , S , P Q =Water discharge S =Water surface or energy slope Channel Slope γ =Specific weight of water Channel slope, a stream’s longitudinal profile, is measured as the difference in Stream power has been defined as the elevation between two points in the product of discharge and slope. Since stream divided by the stream length be- stream discharge typically increases in tween the two points. Slope is one of adownstream direction, slope must the most critical pieces of design infor- decrease in order to minimize stream mation required when channel modifi- power. The decrease in slope in a down- cations are considered. Channel slope stream direction results in the concave- directly impacts flow velocity, stream up longitudinal profile. competence, and stream power. Since Sinuosity is not a profile feature, but it these attributes drive the geomorphic does affect stream slope. Sinuosity is processes of erosion, sediment trans- the stream length between two points port, and sediment deposition, channel on a stream divided by the slope becomes a controlling factor in length between the two points. For channel shape and pattern. example, if a stream is 2,200 feet long Most longitudinal profiles of streams from point Ato point B, and if a valley (See Figs. 1-27 are concave upstream. As described previ- length distance between those two and 1-28) ously in the discussion of dynamic points is 1,000 feet, that stream has a equilibrium, streams adjust their pro- sinuosity of 2.2. A stream can increase file and pattern to try to minimize the its length by increasing its sinuosity, time rate of expenditure of potential resulting in a decrease in slope. This energy, or stream power, present in impact of sinuosity on channel slope flowing water. The concave upward must always be considered if channel shape of a stream’s profile appears to reconstruction is part of a proposed be due to adjustments a river makes restoration. tohelp minimize stream power in a Pools and Riffles downstream direction. Yang (1983) applied the theory of minimum stream The longitudinal profile is seldom power to explain why most longitudinal constant, even over a short reach. Dif- streambed profiles are concave upward. ferences in geology, vegetation pat- In order to satisfy the theory of mini- terns, or human disturbances can mum stream power, which is a special result in flatter and steeper reaches case of the general theory of minimum within an overall profile. Riffles occur

2–22 Chapter 2: Stream Corridor Processes, Characteristics, and Functions where the stream bottom is higher rel- The importance of the bankfull channel ative to streambed elevation immedi- has been established. Channel cross sec- ately upstream or downstream. These tions need to include enough points to relatively deeper areas are considered define the channel in relation to a por- pools. At normal flow, flow velocities tion of the floodplain on each side. A decrease in pool areas, allowing fine suggested guide is to include at least one grained deposition to occur, and in- stream width beyond the highest point crease atop riffles due to the increased on each for smaller stream corri- bed slope between the crest and dors and at least enough of the flood- the subsequent pool. plain on larger streams to clearly define its character in relation to the channel. Longitudinal Profile Adjustments A common example of profile adjust- In meandering streams, the channel ment occurs when a is constructed cross section should be measured in on a stream. The typical response to areas of riffles or crossovers. A riffle or dam construction is channel degrada- crossover occurs between the apexes of tion downstream and up- two sequential . The effects of stream. However, the specific response differences in resistance to erosion of is quite complex as can be illustrated by soil layers are prominent in the outside considering Lane’s relation. typi- bends of meanders, and point bars on cally reduce peak discharges and sedi- the insides of the meanders are con- ment supply in the downstream reach. stantly adjusting to the water and sedi- According to Lane’s relation, a decrease ment loads being moved by the stream. in discharge (Q) should be offset by an The stream’s cross section changes much increase in slope, yet the decrease in more rapidly and frequently in the me- sediment load (Q ) should cause a de- ander bends. There is more variability s crease in slope. This response could be in pool cross sections than in riffle further complicated if armoring occurs cross sections. The cross section in the (D +), which would also cause an in- crossover or riffle area is more uniform. 50 crease in slope. Impacts are not limited Resistance to Flow and Velocity to the main channel, but can include Channel slope is an important factor in aggradation or degradation on tribu- determining streamflow velocity. Flow taries as well. Aggradation often occurs velocity is used to help predict what at the mouths of down- discharge a cross section can convey. As Figure 2.16: stream of dams (and sometimes in the discharge increases, either flow velocity, Channel cross sec- tion. Information entire channel) due to the reduction of flow area, or both must increase. peak flows on the . Obvi- to record when ously, the ultimate response will be the collecting stream cross section data. result of the integration of all these variables. topographic floodplain

Channel Cross Sections hydrologic floodplain Figure 2.16 presents the type of infor- mation that should be recorded when bankfull width collecting stream cross section data. In stable alluvial streams, the high points bankfull elevation on each bank represent the top of the bankfull depth bankfull channel.

Geomorphic Processes 2–23 Roughness plays an important r ole in fore, depends on the bedforms present streams. It helps determine the depth or when that discharge occurs. stage of flow in a stream reach. As flow Vegetation can also contribute to rough- velocity slows in a stream reach due to ness. In streams with boundaries con- roughness, the depth of flow has to in- sisting of cohesive soils, vegetation is crease to maintain the volume of flow usually the principal component of that entered the upstream end of the roughness. The type and distribution of reach (a concept known as flow conti- vegetation in a stream corridor depends nuity). Typical roughness along the on hydrologic and geomorphic boundaries of the stream includes the processes, but by creating roughness, following: vegetation can alter these processes and Sediment particles of different sizes. cause changes in a stream’s form and pattern. Bedforms. Meandering streams offer some resis- Bank irregularities. tance to flow relative to straight The type, amount, and distribution streams. Straight and meandering of living and dead vegetation. streams also have different distributions Other obstructions. of flow velocity that are affected by the alignment of the stream, as shown in Roughness generally increases with in- Figure 2.17. In straight reaches of a creasing particle size. The shape and stream, the fastest flow occurs just size of instream sediment deposits, or below the surface near the center of the bedforms, also contribute to roughness. channel where flow resistance is lowest Sand-bottom streams are good exam- (see Figure 2.17 (a) Section G). In me- ples of how bedform roughness anders, velocities are highest at the out- changes with discharge. At very low dis- side edge due to angular momentum charges, the bed of a sand stream may (see Figure 2.17 (b) Section 3). The dif- be dominated by ripple bedforms. As ferences in flow velocity distribution in flow increases even more, sand dunes meandering streams result in both ero- may begin to appear on the bed. Each sion and deposition at the meander of these bedforms increases the rough- bend. Erosion occurs at the outside of ness of the stream bottom, which tends bends (cutbanks) from high velocity to slow velocity. flows, while the slower velocities at the The depth of flow also increases due to insides of bends cause deposition on increasing roughness. If discharge con- the point (which also has been tinues to increase, a point is reached called the slip-off slope). when the flow velocity mobilizes the The angular momentum of flow sand on the streambed and the entire through a meander bend increases the bed converts again to a planar form. height or super elevationat the outside The depth of flow may actually decrease of the bend and sets up a secondary at this point due to the decreased of flow down the face of the roughness of the bed. If discharge in- cutbank and across the bottom of the creases further still, antidunes may pool toward the inside of the bend. This form. These bedforms create enough rotating flow is called helical flowand friction to again cause the flow depth to the direction of rotation is illustrated increase. The depth of flow for a given on the diagram on the following page by discharge in sand-bed streams, there- the arrows at the top and bottom of cross sections 3 and 4 in the figure.

2–24 Chapter 2: Stream Corridor Processes, Characteristics, and Functions (a) (b)

0

1 Section C 2

0 1 helical flow 1

2 Section E 3 Depth (feet) helical flow 0 2 1 1 helical flow 2 2 Section G 3 3 4

ow 5 024681012141618 l fl elica Horizontal Distance (feet) h 3

helical flow high low velocity velocity

Figure 2.17: Velocity distribution in a (a) straight stream branch and a (b) stream w helical flo Generalized Surface meander. Stream flow velocities are different Streamlines through pools and riffles, in straight and 4 curved reaches, across the stream at any point, and at different depths. Velocity distribution also differs dramatically from baseflow condi- tions through bankfull flows, and flood flows. Source: Leopold et al. 1964. Published by permission of Dover Publications. Generalized Velocity Distributions 5 The distribution of flow velocities in straight and meandering streams is im- predators in pools. Riffle areas are not portant to understand when planning as deep as pools, so more turbulent and designing modifications in stream flows occur in these shallow zones. The alignment in a stream corridor restora- turbulent flow can increase the dis- tion. Areas of highest velocities generate solved oxygen content of the water and the most stream power, so where such may also increase the oxidation and velocities intersect the stream bound- volatilization of some chemical con- aries indicates where more durable pro- stituents in water. tection may be needed. Another extremely important function As flow moves through a meander, the of roughness elements is that they cre- bottom water and detritus in the pool ate aquatic habitat. As one example, are rotated to the surface. This rotation the deepest flow depths usually occur is an important mechanism in moving at the base of cutbanks. These scour drifting and benthic organisms past holes or pools create very different

Geomorphic Processes 2–25 habitat than occurs in the depositional sediment from the watershed. Vertical environment of the slip-off slope. accretion is the deposition of sediment on flooded surfaces. This sediment Active Channels and generally is finer textured than point Floodplains bar sediments and is considered to be Floodplains are built by two stream an overbank deposit. Vertical accretion processes, lateral and vertical accretion. occurs on top of the lateral accretion Lateral accretion is the deposition of deposits in the point bars; however, sediment on point bars on the insides lateral accretion is the dominant of bends of the stream. The stream lat- process. It typically makes up 60 to 80 erally migrates across the floodplain as percent of the total sediment deposits the outside of the meander bend in floodplains (Leopold et al. 1964). erodes and the builds with It is apparent that lateral migration of coarse-textured sediment. This naturally meanders is an important natural occurring process maintains the cross process since it plays a critical role in section needed to convey water and reshaping floodplains.

2.C Physical and Chemical Characteristics

The quality of water in the stream corri- a few key concepts that are relevant to dor might be a primary objective of stream corridor restoration. The reader restoration, either to improve it to a de- is referred to other sources (e.g., sired condition or to sustain it. Estab- Thomann and Mueller 1987, Mills et al. lishing an appropriate flow regime and 1985) for a more detailed treatment. geomorphology in a stream corridor As in the previous sections, the physical may do little to ensure a healthy ecosys- and chemical characteristics of streams tem if the physical and chemical charac- are examined in both the lateral and teristics of the water are inappropriate. longitudinal perspectives. The lateral For example, a stream containing high perspective refers to the influence of the concentrations of toxic materials or in watershed on water quality, with partic- which high temperatures, low dissolved ular attention to riparian areas. The lon- oxygen, or other physical/chemical gitudinal perspective refers to processes characteristics are inappropriate cannot that affect water quality during trans- support a healthy stream corridor. Con- port instream. versely, poor condition of the stream corridor—such as lack of riparian shad- Physical Characteristics ing, poor controls on erosion, or exces- sive sources of nutrients and oxygen- Sediment demanding waste—can result in degra- Section 2.B discussed total sediment dation of the physical and chemical loads in the context of the evolution of conditions within the stream. stream form and geomorphology. In ad- This section briefly surveys some of the dition to its role in shaping stream key physical and chemical characteristics form, suspended sediment plays an im- of flowing waters. Stream water quality portant role in water quality, both in is a broad topic on which many books the water column and at the sediment- have been written. The focus here is on water interface. In a water quality con-

2–26 Chapter 2: Stream Corridor Processes, Characteristics, and Functions text, sediment usually refers to soil par- Sediment Across the Stream Corridor ticles that enter the water column from Rain erodes and washes soil particles eroding land. Sediment consists of par- off plowed fields, construction sites, ticles of all sizes, including fine clay logging sites, urban areas, and strip- particles, silt, and gravel. The term sedi- mined lands into waterbodies. Eroding mentation is used to describe the depo- streambanks also deposit sediment into sition of sediment particles in waterbodies. In sum, sediment quality waterbodies. in the stream represents the net result Although sediment and its transport of erosion processes in the watershed. occur naturally in any stream, changes The lateral view of sediment is dis- in sediment load and particle size can cussed in more detail in Section 2.B. have negative impacts (Figure 2.18). It is worth noting, however, that from Fine sediment can severely alter aquatic a water quality perspective, interest may communities. Sediment may clog and focus on specific fractions of the sedi- abrade fish gills, suffocate eggs and ment load. For instance, controlling aquatic insect larvae on the bottom, fine sediment load is often of particular and fill in the pore space between bot- concern for restoration of habitat for tom cobbles where fish lay eggs. Sedi- salmonid fish. ment interferes with recreational activities and aesthetic enjoyment at Restoration efforts may be useful for waterbodies by reducing water clarity controlling loads of sediment and sedi- and filling in waterbodies. Sediment ment-associated pollutants from the also may carry other pollutants into wa- watershed to streams. These may range terbodies. Nutrients and toxic chemicals from efforts to reduce upland erosion may attach to sediment particles on to treatments that reduce sediment de- land and ride the particles into surface livery through the . Design waters where the pollutants may settle of restoration treatments is covered in with the sediment or become soluble in Chapter 8. the water column. Studies have shown that fine sediment intrusion can significantly impact the quality of spawning habitat (Cooper 1965, Chapman 1988). Fine sediment intrusion into streambed gravels can re- duce permeability and intragravel water velocities, thereby restricting the supply of oxygenated water to developing salmonid embryos and the removal of their metabolic wastes. Excessive fine sediment deposition can effectively smother incubating eggs and entomb alevins and fry. A sediment intrusion model (Alonso et al. 1996) has been developed, verified, and validated to predict the within-redd (spawning area) sediment accumulation and dissolved Figure 2.18: Stream sedimentation. Although oxygen status. sediment and its transport occur naturally, changes in sediment load and particle size have negative impacts.

Physical and Chemical Characteristics 2–27 Sediment Along the Stream Corridor dresses the effects of the temperature of The longitudinal processes affecting influent water. FAST sediment transport from a water quality The most important factor for tempera- FORWARD perspective are the same as those dis- ture of influent water within a stream cussed from a geomorphic perspective reach is the balance between water ar- in Section 2.B. As in the lateral perspec- riving via surface and ground water tive, interest from a water quality point pathways. Water that flows over the Preview Sec- of view may be focused on specific sedi- land surface to a stream has the oppor- tion D for ment size fractions, particularly the fine tunity to gain heat through contact with more detail on sediment fraction, because of its effect surfaces heated by the sun. In contrast, the effects of on water quality, water temperature, ground water is usually cooler in sum- cover on water habitat, and biota. mer and tends to reflect average annual temperature. temperatures in the watershed. Water Water Temperature flow via shallow ground water pathways Water temperature is a crucial factor in may lie between the average annual stream corridor restoration for a number temperature and ambient temperatures of reasons. First, dissolved oxygen solu- during runoff events. bility decreases with increasing water Both the fraction of runoff arriving via temperature, so the stress imposed by surface pathways and the temperature oxygen-demanding waste increases with of surface runoff are strongly affected higher temperatures. Second, tempera- by the amount of impervious surfaces ture governs many biochemical and within a watershed. For example, hot physiological processes in cold-blooded paved surfaces in a watershed can heat aquatic organisms, and increased tem- surface runoff and significantly increase peratures can increase metabolic and the temperature of streams that receive reproductive rates throughout the food the runoff. chain. Third, many aquatic species can Water Temperature Along the tolerate only a limited range of tempera- Stream Corridor tures, and shifting the maximum and minimum temperatures within a stream Water also is subject to thermal loading can have profound effects on species through direct effects of sunlight on composition. Finally, temperature also streams. For the purposes of restoration, affects many abiotic chemical processes, land use practices that remove overhead such as reaeration rate, sorption of or- cover or that decrease can in- ganic chemicals to particulate matter, crease instream temperatures to levels and volatilization rates. Temperature in- that exceed critical thermal maxima for creases can lead to increased stress from fishes (Feminella and Matthews 1984). toxic compounds, for which the dis- Maintaining or restoring normal tem- solved fraction is usually the most perature ranges can therefore be an im- bioactive fraction. portant goal for restoration. Water Temperature Across the Chemical Constituents Stream Corridor Previous chapters have discussed the Water temperature within a stream physical journey of water as it moves reach is affected by the temperature of through the hydrologic cycle. Rain per- water upstream, processes within the colates to the ground water table or be- stream reach, and the temperature of comes overland flow, streams collect influent water. The lateral view ad- this water and route it toward the

2–28 Chapter 2: Stream Corridor Processes, Characteristics, and Functions ocean, and evapotranspiration occurs Clay Sand throughout the cycle. As water makes this journey, its chemistry changes. While in the air, water equilibrates with atmospheric gases. In shallow soils, it undergoes chemical exchanges with in- organic and organic matter and with organic coating soil gases. In ground water, where transit iron coating times are longer, there are more oppor- tunities for minerals to dissolve. Similar Figure 2.19: The organic coatings on suspend- ed sediment from streams. Water chemistry chemical reactions continue along determines whether sediment will carry stream corridors. Everywhere, water in- adsorbed materials or if stream sediments teracts with everything it touches—air, will be coated. rocks, bacteria, plants, and fish—and is affected by human disturbances. The total concentration of all dissolved ions in water, also known as salinity, Scientists have been able to define sev- varies widely. Precipitation typically eral interdependent cycles for many of contains only a few parts per thousand the common dissolved constituents in (ppt) of dissolved solids, while the water. Central among these cycles is the salinity of seawater averages about 35 behavior of oxygen, carbon, and nutri- ppt (Table 2.5). The concentration of ents, such as nitrogen (N), phosphorus dissolved solids in freshwater may vary (P), sulfur (S), and smaller amounts of from only 10 to 20 mg/L in a pristine common trace elements. mountain stream to several hundred Iron, for example, is an essential ele- mg/L in many rivers. Concentrations ment in the metabolism of animals and may exceed 1,000 mg/L in arid water- plants. Iron in aquatic systems may be sheds. A dissolved solids concentration present in one of two oxidation states. of less than 500 mg/L is recommended Ferric iron (Fe3+) is the more oxidized for public drinking water, but this form and is very sparingly soluble in threshold is exceeded in many areas of water. The reduced form, ferrous iron the country. Some crops (notably fruit (Fe2+), is more soluble by many orders trees and beans) are sensitive to even of magnitude. In many aquatic systems, modest salinity, while other crops, such such as lakes for example, iron can cycle as cotton, barley, and beets, tolerate from the ferric state to the ferrous state high concentrations of dissolved solids. and back again (Figure 2.19). The oxi- Agricultural return water from irrigation dation of ferrous iron followed by the may increase salinity in streams, partic- precipitation of ferric iron results in ularly in the west. Recommended salin- iron coatings on the surfaces of some ity limits for livestock vary from 2,860 stream sediments. These coatings, along mg/L for poultry to 12,900 mg/L for with organic coatings, play a substantial adult sheep. Plants, fish, and other role in the aquatic chemistry of toxic aquatic life also vary widely in their trace elements and toxic organic chemi- adaptation to different concentrations cals. The chemistry of toxic organic of dissolved solids. Most species have a chemicals and metals, along with the maximum salinity tolerance, and few cycling and chemistry of oxygen, nitro- can live in very pure water of very low gen, and phosphorus, will be covered ionic concentration. later in this section.

Physical and Chemical Characteristics 2–29 Samples fluctuations in pH also can stress Constituent 123 4 5 6 aquatic organisms. Finally, acidic condi-

SiO2 tions also can aggravate toxic contami- Al nation problems through increased Fe solubility, leading to the release of toxic Ca chemicals stored in stream sediments. Mg pH, Alkalinity, and Acidity Across the Na Stream Corridor K The pH of runoff reflects the chemical NH4 characteristics of precipitation and the HCO3 land surface. Except in areas with signif- SO4 icant ocean spray, the dominant ion in Cl most precipitation is bicarbonate – NO2 (HCO3 ). The bicarbonate ion is pro-

NO3 duced by carbon dioxide reacting with Total water: dissolved + – solids H2O + CO2 = H + HCO3 pH This reaction also produces a hydrogen 1. Snow, Spooner Summit. U.S. Highway 50, Nevada (east of Lake + Tahoe) (Feth, Rogers, and Roberson, 1964). ion (H ), thus increasing the hydrogen 2. Average composition of rain, August 1962 to July 1963, at 27 points ion concentration and acidity and low- in North Carolina and Virginia (Gambell and Fisher, 1966). 3. Rain, Menlo Park, Calif., 7:00 p.m. Jan. 9 to 8:00 a.m. Jan 10, 1958 ering the pH. Because of the presence (Whitehead and Feth, 1964).

4. Rain, Menlo Park, Calif., 8:00 a.m. to 2:00 p.m. Jan 10, 1958 of CO2 in the atmosphere, most rain is (Whitehead and Feth, 1964). 5. Average for inland sampling stations in the United States for 1 year. naturally slightly acidic, with a pH of Table 2.5: Data from Junge and Werby (1958), as reported by Whitehead and about 5.6. Increased acidity in rainfall Composition, in mil- Feth (1964). 6. Average composition of precipitation, Williamson Creek, Snohomish can be caused by inputs, particularly igrams per liter, of County, Wash., 1973-75. Also reported: As, 0.00045 mg/L; Cu 0.0025 mg/L; Pb, 0.0033 mg/L; Zn, 0.0036 mg/L (Deithier, D.P., 1977, Ph.D. from burning fossil fuels. rain and snow. thesis. University of Washington, Seattle). As water moves through soils and rocks, pH, Alkalinity, and Acidity its pH may increase or decrease as addi- tional chemical reactions occur. The car- Alkalinity, acidity, and buffering capac- bonate buffering system controls the ity are important characteristics of water acidity of most waters. Carbonate that affect its suitability for biota and buffering results from chemical equilib- influence chemical reactions. The acidic rium between calcium, carbonate, bicar- or basic (alkaline) nature of water is bonate, carbon dioxide, and hydrogen commonly quantified by the negative ions in the water and carbon dioxide in logarithm of the hydrogen ion concen- the atmosphere. Buffering causes waters tration, or pH. A pH value of 7 repre- to resist changes in pH (Wetzel 1975). sents a neutral condition; a pH value Alkalinity refers to the acid-neutralizing less than 5 indicates moderately acidic capacity of water and usually refers to conditions; a pH value greater than 9 those compounds that shift the pH in indicates moderately alkaline condi- an alkaline direction (APHA 1995, Wet- tions. Many biological processes, such zel 1975). The amount of buffering is as reproduction, cannot function in related to the alkalinity and primarily acidic or alkaline waters. In particular, determined by carbonate and bicarbon- aquatic organisms may suffer an os- ate concentration, which are introduced motic imbalance under sustained expo- into the water from dissolved calcium sure to low pH waters. Rapid carbonate (i.e., limestone) and similar

2–30 Chapter 2: Stream Corridor Processes, Characteristics, and Functions minerals present in the watershed. For tion also tend to stabilize highly vari- example, when an acid interacts with able pH levels attributable to high rates limestone, the following dissolution of photosynthesis. reaction occurs: The pH within streams can have impor- + 2+ –

H + CaCO3 = Ca + HCO3 tant consequences for toxic materials. This reaction consumes hydrogen ions, High acidity or high alkalinity tend to thus raising the pH of the water. Con- convert insoluble metal sulfides to solu- versely, runoff may acidify when all al- ble forms and can increase the concen- kalinity in the water is consumed by tration of toxic metals. Conversely, high acids, a process often attributed to the pH can promote ammonia toxicity. Am- input of strong mineral acids, such as monia is present in water in two forms, unionized (NH ) and ionized (NH +). sulfuric acid, from acid mine drainage, 3 4 and weak organic acids, such as humic Of these two forms of ammonia, un- and fulvic acids, which are naturally ionized ammonia is relatively highly produced in large quantities in some toxic to aquatic life, while ionized am- types of soils, such as those associated monia is relatively negligibly toxic. The with coniferous forests, bogs, and wet- proportion of un-ionized ammonia is lands. In some streams, pH levels can determined by the pH and temperature be increased by restoring degraded wet- of the water (Bowie et al. 1985)—as pH lands that intercept acid inputs, such as or temperature increases, the propor- acid mine drainage, and help neutralize tion of un-ionized ammonia and the acidity by converting sulfates from sul- toxicity also increase. For example, with furic acid to insoluble nonacidic metal a pH of 7 and a temperature of 68°F, sulfides that remain trapped in wetland only about 0.4 percent of the total am- sediments. monia is in the un-ionized form, while at a pH of 8.5 and a temperature of pH, Alkalinity, and Acidity Along the 78°F, 15 percent of the total ammonia Stream Corridor is in the un-ionized form, representing Within a stream, similar reactions occur 35 times greater potential toxicity to between acids in the water, atmospheric aquatic life. CO , alkalinity in the water column, and 2 Dissolved Oxygen streambed material. An additional char- acteristic of pH in some poorly buffered Dissolved oxygen (DO) is a basic re- waters is high daily variability in pH lev- quirement for a healthy aquatic ecosys- els attributable to biological processes tem. Most fish and aquatic insects that affect the carbonate buffering sys- “breathe” oxygen dissolved in the water tem. In waters with large standing crops column. Some fish and aquatic organ- of aquatic plants, uptake of carbon diox- isms, such as carp and sludge worms, ide by plants during photosynthesis re- are adapted to low oxygen conditions, moves carbonic acid from the water, but most sport fish species, such as which can increase pH by several units. trout and salmon, suffer if DO concen- Conversely, pH levels may fall by several trations fall below a concentration of 3 units during the night when photosyn- to 4 mg/L. Larvae and juvenile fish are thesis does not occur and plants give off more sensitive and require even higher carbon dioxide. Restoration techniques concentrations of DO (USEPA 1997). that decrease instream plant growth Many fish and other aquatic organisms through increased shading or reduction can recover from short periods of low in nutrient loads or that increase reaera-

Physical and Chemical Characteristics 2–31 DO in the water. However, prolonged equivalent indicator rather than a true episodes of depressed dissolved oxygen physical or chemical substance. It mea-

FAST concentrations of 2 mg/L or less can re- sures the total concentration of DO that FORWARD sult in “dead” waterbodies. Prolonged eventually would be demanded as exposure to low DO conditions can suf- wastewater degrades in a stream. focate adult fish or reduce their repro- BOD also is often separated into car- ductive survival by suffocating sensitive bonaceous and nitrogenous compo- Preview Section eggs and larvae, or can starve fish by nents. This is because the two fractions D for more in- killing aquatic insect larvae and other tend to degrade at different rates. Many formation on prey. Low DO concentrations also favor water quality models for dissolved oxy- DO. anaerobic bacteria that produce the gen require as input estimates of ulti- noxious gases or foul odors often asso- mate carbonaceous BOD (CBODu) and ciated with polluted waterbodies. either ultimate nitrogenous BOD

Water absorbs oxygen directly from the (NBODu) or concentrations of individ- atmosphere, and from plants as a result ual nitrogen species. of photosynthesis. The ability of water Oxygen-demanding wastes can be to hold oxygen is influenced by temper- loaded to streams by point source dis- ature and salinity. Water loses oxygen charges, nonpoint loading, and ground primarily by respiration of aquatic water. BOD loads from major point plants, animals, and microorganisms. sources typically are controlled and Due to their shallow depth, large sur- monitored and thus are relatively easy face exposure to air, and constant mo- to analyze. Nonpoint source loads of tion, undisturbed streams generally BOD are much more difficult to ana- contain an abundant DO supply. How- lyze. In general, any loading of organic ever, external loads of oxygen-demand- material from a watershed to a stream ing wastes or excessive plant growth results in an oxygen demand. Excess induced by nutrient loading followed loads of organic material may arise by death and decomposition of vegeta- from a variety of land use practices, tive material can deplete oxygen. coupled with storm events, erosion, Dissolved Oxygen Across the and washoff. Some agricultural activi- Stream Corridor ties, particularly large-scale animal Oxygen concentrations in the water col- operations and improper manure appli- umn fluctuate under natural conditions, cation, can result in significant BOD but oxygen can be severely depleted as loads. Land-disturbing activities of silvi- a result of human activities that intro- culture and construction can result in duce large quantities of biodegradable high organic loads through the erosion organic materials into surface waters. of organic topsoil. Finally, Excess loading of nutrients also can de- often is loaded with high concentra- plete oxygen when plants within a tions of organic materials derived from stream produce large quantities of plant a variety of sources. biomass. Dissolved Oxygen Along the Loads of oxygen-demanding waste usu- Stream Corridor ally are reported as biochemical oxygen Within a stream, DO content is affected demand (BOD). BOD is a measure of by reaeration from the atmosphere, pro- the amount of oxygen required to oxi- duction of DO by aquatic plants as a dize organic material in water by bio- by-product of photosynthesis, and con- logical activity. As such, BOD is an sumption of DO in respiration by

2–32 Chapter 2: Stream Corridor Processes, Characteristics, and Functions plants, animals, and, most importantly, reaeration. In general, oxygen transfer microorganisms. in natural waters depends on the fol- Major processes affecting the DO bal- lowing: ance within a stream are summarized in Internal mixing and turbulence due Figure 2.20. This includes the following to velocity gradients and fluctuation components: Temperature Carbonaceous deoxygenation Wind mixing Nitrogenous deoxygenation , dams, and (nitrification) Surface films Reaeration Water column depth. Sediment oxygen demand

Photosynthesis and respiration Figure 2.20: Interrelationship of major kinetic of plants. processes for BOD and DO as represented by water quality models. Complex, interacting Reaeration is the primary route for in- physical and chemical processes can sometimes troducing oxygen into most waters. be simplified by models in order to plan a

Oxygen gas (O2) constitutes about 21 restoration. percent of the atmosphere and readily dissolves in water. The saturation con- centration of DO in water is a measure of the maximum amount of oxygen that water can hold at a given tempera- ture. When oxygen exceeds the satura- carbonaceous tion concentration, it tends to degas to deoxygenation

atmospheric settling + oxygen reaeration the atmosphere. When oxygen is below NH4 the saturation concentration, it tends to diffuse from the atmosphere to water. - nitrification The saturation concentration of oxygen NO2 oxygen demand decreases with temperature according to a complex power function equation - (APHA 1995). In addition to tempera- NO3 ture, the saturation concentration is af- fected by water salinity and the gen v ed ox y atmospheric pressure. As the salinity of s ol is water increases, the saturation concen- d tration decreases. As the atmospheric pressure increases the saturation con- respiration centration also increases. Interactions between atmospheric and DO are driven by the partial pressure gradient in the gas phase and the con- centration gradient in the liquid phase photosynthesis algae (Thomann and Mueller 1987). Turbu- lence and mixing in either phase de- crease these gradients and increase reaeration, while a quiescent, stagnant surface or films on the surface reduce

Physical and Chemical Characteristics 2–33 Stream restoration techniques often (e.g., Thomann and Mueller 1987), and take advantage of these relationships, a variety of well-tested computer mod- for instance by the installation of artifi- els are available. Most stream water cial cascades to increase reaeration. quality models account for CBOD in Many empirical formulations have been the water column separately from developed for estimating stream reaera- NBOD (which is usually represented tion rate coefficients; a detailed sum- via direct mass balance of nitrogen mary is provided in Bowie et al. (1985). species) and sediment oxygen demand or In addition to reaeration, oxygen is pro- SOD. SOD represents the oxygen de- duced instream by aquatic plants. mand of sediment organism respiration Through photosynthesis, plants capture and the benthic decomposition of or- energy from the sun to fix carbon diox- ganic material. The demand of oxygen ide into reduced organic matter: by sediment and benthic organisms can, in some instances, be a significant

6 CO2 + 6 H2O = C6H12O6 + 6 O2 fraction of the total oxygen demand in Note that photosynthesis also produces a stream. This is particularly true in oxygen. Plants utilize their simple pho- small streams. The effects may be par- tosynthetic sugars and other nutrients ticularly acute during low-flow and (notably nitrogen [N], phosphorus [P], high-temperature conditions, as micro- and sulfur [S] with smaller amounts of bial activity tends to increase with in- several common and trace elements) to creased temperature. operate their metabolism and to build The presence of toxic pollutants in the their structures. water column can indirectly lower oxy- Most animal life depends on the release gen concentrations by killing algae, of energy stored by plants in the photo- aquatic weeds, or fish, which provide synthetic process. In a reaction that is an abundance of food for oxygen- the reverse of photosynthesis, animals consuming bacteria. Oxygen depletion consume plant material or other ani- also can result from chemical reactions mals and oxidize the sugars, starches, that do not involve bacteria. Some pol- and proteins to fuel their metabolism lutants trigger chemical reactions that and build their own structure. This place a on process is known as respiration and receiving waters. consumes dissolved oxygen. The actual Nutrients process of respiration involves a series of energy converting oxidation-reduc- In addition to carbon dioxide and tion reactions. Higher animals and water, aquatic plants (both algae and many microorganisms depend on suffi- higher plants) require a variety of other cient dissolved oxygen as the terminal elements to support their bodily struc- electron acceptor in these reactions and tures and metabolism. Just as with ter- cannot survive without it. Some mi- restrial plants, the most important of croorganisms are able to use other com- these elements are nitrogen and phos- pounds (such as nitrate and sulfate) as phorus. Additional nutrients, such as electron acceptors in metabolism and potassium, iron, selenium, and silica, can survive in anaerobic (oxygen- are needed in smaller amounts and depleted) environments. generally are not limiting factors to Detailed information on analysis and plant growth. When these chemicals are modeling of DO and BOD in streams limited, plant growth may be limited. is contained in a number of references This is an important consideration in

2–34 Chapter 2: Stream Corridor Processes, Characteristics, and Functions stream management. Plant biomass said to be the limiting nutrient on plant (either created instream or loaded from growth. In streams experiencing exces- the watershed) is necessary to support sive nutrient loading, resource man- the food chain. However, excessive agers often seek to control loading of growth of algae and other aquatic the limiting nutrient at levels that pre- plants instream can result in nuisance vent nuisance conditions. conditions, and the depletion of dis- In the aquatic environment, nitrogen solved oxygen during nonphotosyn- can exist in several forms—dissolved ni- thetic periods by the respiration of trogen gas (N2), ammonia and ammo- plants and decay of dead plant material + nium ion (NH3 and NH4 ), nitrite can create conditions unfavorable to – – (NO2 ), nitrate (NO3 ), and organic ni- aquatic life. trogen as proteinaceous matter or in Phosphorus in freshwater systems exists dissolved or particulate phases. The in either a particulate phase or a dis- most important forms of nitrogen in solved phase. Both phases include or- terms of their immediate impacts on ganic and inorganic fractions. The water quality are the readily available organic particulate phase includes living ammonia ions, nitrites, and nitrates. Be- and dead particulate matter, such as cause they must be converted to a form plankton and detritus. Inorganic partic- more usable by plants, particulate and ulate phosphorus includes phosphorus organic nitrogen are less important in precipitates and phosphorus adsorbed the short term. to particulates. Dissolved organic phos- It may seem unusual that nitrogen phorus includes organic phosphorus could limit plant growth, given that the excreted by organisms and colloidal atmosphere is about 79 percent nitro- phosphorus compounds. The soluble gen gas. However, only a few life-forms inorganic phosphate forms H PO –, 2 4 (for example, certain bacteria and blue- HPO 2–, and PO 3–, collectively known 4 4 green algae) have the ability to fix nitro- as soluble reactive phosphorus (SRP) are gen gas from the atmosphere. Most readily available to plants. Some con- plants can use nitrogen only if it is densed phosphate forms, such as those available as ammonia (NH3, commonly found in detergents, are inorganic but present in water as the ionic form am- are not directly available for plant up- + – monium, NH4 ) or as nitrate (NO3 ) take. Aquatic plants require nitrogen (Figure 2.21). However, in freshwater and phosphorus in different amounts. systems, growth of aquatic plants is For phytoplankton, as an example, cells more commonly limited by phospho- µ contain approximately 0.5 to 2.0 g rus than by nitrogen. This limitation oc- µ phosphorus per g chlorophyll, and 7 curs because phosphate (PO 3–) forms µ µ 4 to 10 g nitrogen per g chlorophyll. insoluble complexes with common From this relationship, it is clear that constituents in water (Ca++ and variable the ratio of nitrogen and phosphorus amounts of OH–, Cl–, and F–). Phospho- required is in the range of 5 to 20 rus also sorbs to iron coatings on clay (depending on the characteristics of and other sediment surfaces and is individual species) to support full therefore removed from the water col- utilization of available nutrients and umn by chemical processes, resulting in maximize plant growth. When the the reduced ability of the water body to ratio deviates from this range, plants support plant growth. cannot use the nutrient present in ex- cess amounts. The other nutrient is then

Physical and Chemical Characteristics 2–35 riparian vegetation atmospheric N2

se litter inputs dim e dissolved n t organic s u dissolved export to nitrogen r f import from a organic downstream NH3 c upstream nitrogen e stream water NO NH 3 interstitial 3 water NO3 assimilation oxygen concent- nitrogen ration fixation NO3 nitrification N2 biota assimilation cyanobacteria NO2 NH3 and microbial populations decomposition NH benthic algae 3 NO3 excretion assimilation

N2 nitrogen particulate fixation organic matter NO2 and associated decomposition NH3 microbes accum- ulation excretion

denitrification NH3

ground water dissolved organic nitrogen NO3

Figure 2.21: Dynamics and transformations of nitrogen in a stream ecosystem. Nutrient cycling from one form to another occurs with changes in nutrient inputs, as well as temperature and oxygen available.

Nutrients Across the Stream Corridor is the direct discharge of treated waste Both nitrogen and phosphorus are from plants, as delivered to surface waters at an ele- well as combined sewer overflows vated rate as a result of human activi- (CSOs). Such point source discharges ties, including point source discharges are regulated under the National Pollu- of treated wastewater and nonpoint tant Discharge Elimination System sources, such as agriculture and urban (NPDES) and usually are well character- development. In many developed wa- ized by monitoring. The NPDES re- tersheds, a major source of nutrients quires permitted dischargers to meet

2–36 Chapter 2: Stream Corridor Processes, Characteristics, and Functions both numeric and narrative water qual- Because of its tendency to sorb to sedi- ity standards in streams. While most ment particles and organic matter, states do not have numeric standards phosphorus is transported primarily in for nutrients, point source discharges surface runoff with eroded sediments. of nutrients are recognized as a factor Inorganic nitrogen, on the other hand, leading to stream degradation and fail- does not sorb strongly and can be trans- ure to achieve narrative water quality ported in both particulate and dissolved standards. As a result, increasingly strin- phases in surface runoff. Dissolved in- gent limitations on nutrient concentra- organic nitrogen also can be trans- tions in wastewater treatment plant ported through the unsaturated zone effluent (particularly phosphorus) have (interflow) and ground water to water- been imposed in many areas. bodies. Table 2.6 presents common In many cases the NPDES program has point and nonpoint sources of nitrogen significantly cleaned up rivers and and phosphorus loading and shows the streams; however, many streams still do approximate concentrations delivered. not meet water quality standards, even Note that nitrates are naturally occur- with increasingly stringent regulatory ring in some soils. standards. Scientists and regulators now Nutrients Along the Stream Corridor understand that the dominant source of Nitrogen, because it does not sorb nutrients in many streams is from non- strongly to sediment, moves easily be- point sources within the stream’s water- tween the substrate and the water col- shed, not from point sources such as umn and cycles continuously. Aquatic wastewater treatment plants. Typical organisms incorporate dissolved and land uses that contribute to the non- particulate inorganic nitrogen into pro- point contamination of streams are the teinaceous matter. Dead organisms de- application of fertilizers to agricultural compose and nitrogen is released as fields and suburban lawns, the improper ammonia ions and then converted to handling of animal wastes from live- nitrite and nitrate, where the process stock operations, and the disposal of begins again. human waste in septic systems. Storm runoff from agricultural fields can con- Phosphorus undergoes continuous tribute nutrients to a stream in dissolved transformations in a freshwater envi- forms as well as particulate forms. ronment. Some phosphorus will sorb to

Table 2.6: Sources and concentrations of pollutants from common point and nonpoint sources.

Source Total Nitrogen (mg/L) Total Phosphorus (mg/L) Urban runoffa 3–10 0.2–1.7 Livestock operationsa 6–800b 4–5 Atmosphere (wet deposition)a 0.9 0.015c 90% forestd 0.06–0.19 0.006–0.012 50% forestd 0.18–0.34 0.013–0.015 90% agricultured 0.77–5.04 0.085–0.104 Untreated wastewatera 35 10 Treated wastewatera,e 30 10 a Novotny and Olem (1994). b As organic nitrogen. c Sorbed to airborne particulate. d Omernik (1987). e With .

Physical and Chemical Characteristics 2–37 sediments in the water column or sub- The movement of organic chemicals strate and be removed from circulation. from the watershed land surface to a The SRP (usually as orthophosphate) is water body is largely determined by the assimilated by aquatic plants and con- characteristics of the chemical, as dis- verted to organic phosphorus. Aquatic cussed below under the longitudinal plants then may be consumed by detri- perspective. Pollutants that tend to sorb tivores and grazers, which in turn ex- strongly to soil particles are primarily crete some of the organic phosphorus transported with eroded sediment. Con- as SRP. Continuing the cycle, the SRP is trolling sediment delivery from source rapidly assimilated by aquatic plants. area land uses is therefore an effective management strategy. Organic chemi- Toxic Organic Chemicals cals with significant solubility may be Pollutants that cause toxicity in animals transported directly with the flow of or humans are of obvious concern to water, particularly stormflow from im- restoration efforts. Toxic organic chemi- pervious urban surfaces. cals (TOC) are synthetic compounds Toxic Organic Chemicals Along the that contain carbon, such as polychlori- Stream Corridor nated biphenyls (PCBs) and most pesti- Among all the elements of the earth, cides and herbicides. Many of these carbon is unique in its ability to form a synthesized compounds tend to persist virtually infinite array of stable covalent and accumulate in the environment be- bonds with itself: long chains, branches cause they do not readily break down and rings, spiral helixes. Carbon mole- in natural ecosystems. Some of the cules can be so complex that they are most toxic synthetic organics, DDT and able to encode information for the orga- PCBs, have been banned from use in nization of other carbon structures and the United States for decades yet con- the regulation of chemical reactions. tinue to cause problems in the aquatic ecosystems of many streams. The chemical industry has exploited this to produce many useful organic Toxic Organic Chemicals Across the chemicals: plastics, paints and dyes, Stream Corridor fuels, pesticides, pharmaceuticals, and TOCs may reach a water body via both other items of modern life. These prod- point and nonpoint sources. Because ucts and their associated wastes and by- permitted NPDES point sources must products can interfere with the health meet water quality standards instream of aquatic ecosystems. Understanding and because of whole effluent toxicity the transport and fate of synthetic or- requirements, continuing TOC prob- ganic compounds (SOC) in aquatic envi- lems in most streams are due to non- ronments continues to challenge point loading, recycling of materials scientists. Only a general overview of stored in stream and riparian sedi- the processes that govern the behavior ments, , or accidental of these chemicals along stream corri- spills. Two important sources of non- dors is presented here. point loading of organic chemicals are Solubility application of pesticides and herbicides in connection with agriculture, silvicul- It is the nature of the carbon-carbon ture, or suburban lawn care, and runoff bond that electrons are distributed rela- from potentially polluted urban and in- tively uniformly between the bonded dustrial land uses. atoms. Thus a chained or ringed hydro- carbon is a fairly nonpolar compound.

2–38 Chapter 2: Stream Corridor Processes, Characteristics, and Functions This nonpolar nature is dissimilar to refers to the delocalized bonding struc- the molecular structure of water, which ture of a ringed compound like ben- is a very polar solvent. zene (Figure 2.23). (Indeed, all On the general principle that “like dis- aromatic compounds can be considered solves like,” dissolved constituents in derivatives of benzene.) Because elec- water tend to be polar. Witness, for ex- trons are free to “dance around the ample, the ionic nature of virtually all ring” of the benzene molecule, benzene inorganic constituents discussed thus and its derivatives are more compatible far in this chapter. How does an organic with the polar nature of water. compound become dissolved in water? A simple example will illustrate the There are several ways. The compound factors enhancing aqueous solubility of can be relatively small, so it minimizes organic compounds. Six compounds, its disturbance of the polar order of each having six carbons, are shown in things in aqueous solution. Alterna- Table 2.7. Hexane is a simple hydrocar- tively, the compound may become bon, an alkane whose solubility is 10 more polar by adding polar functional mg/L. Simply by adding a single -OH groups (Figure 2.22). Alcohols are or- group, which converts hexane to the al- ganic compounds with -OH groups at- cohol hexanol, solubility is increased to tached; organic acids are organic 5,900 mg/L. You can bend hexane into compounds with attached -COOH a ringed alkane structure called cyclo- groups. These functional groups are hexane. Forming the ring makes cyclo- highly polar and increase the solubility hexane smaller than hexane and of any organic compound. Even more increases its solubility, but only to 55 solubility in water is gained by ionic mg/L. Making the ring aromatic by - functional groups, such as -COO . forming the six-carbon benzene mole- Another way that solubility is enhanced cule increases solubility all the way to is by increased aromaticity. Aromaticity 1,780 mg/L. Adding an -OH to benzene to form a phenol leads to another dra-

Figure 2.22: Relative aqueous solubility of different functional groups. The solubility of a contaminant in water largely determines the extent to which it will impact water quality.

C-O-C ether O = ester C-O-R O = carbonyl -C- O = carboxyl -C-OH

-OH hydroxyl

-NH amine 2 O = carboxylate -C-O

1 10 100 1,000 10,000 Relative Aqueous Solubility

Physical and Chemical Characteristics 2–39 H H tanol separate (neither is very soluble in the other), and the concentration of the C C H C C H H C C H organic compound can be measured in each phase. The octanol-water partition H C C H H C C H coefficient, or Kow, is defined simply as: C C

Kow = concentration in octanol / H H concentration in water The relation between water solubility Figure 2.23: Aromatic hydrocarbons. Benzene is soluble in water because of its “aromatic” and Kow is shown in Figure 2.24. Gener- structure. ally we see that very insoluble com- pounds like DDT and PCBs have very

high values of Kow. Alternatively, organic matic increase in solubility (to 82,000 acids and small organic solvents like mg/L). Adding a chloride atom to the TCE are relatively soluble and have low K values. benzene ring diminishes its aromatic ow character (chloride inhibits the dancing The octanol-water partition coefficient electrons), and thus the solubility of has been determined for many com- chlorobenzene (448 mg/L) is less than pounds and can be useful in under- benzene. standing the distribution of SOC Sorption between water and biota, and between water and sediments. Compounds with In the 1940s, a young pharmaceutical high Kow tend to accumulate in fish industry sought to develop medicines tissue (Figure 2.25). The sediment-water that could be transported in digestive distribution coefficient, often expressed fluids and blood (both of which are as Kd, is defined in a sediment-water essentially aqueous solutions) and mixture at equilibrium as the ratio of could also diffuse across cell mem- the concentration in the sediment to branes (which have, in part, a rather the concentration in the water: nonpolar character). The industry devel- K = concentration in sediment / oped a parameter to quantify the polar d versus nonpolar character of potential concentration in water drugs, and they called that parameter One might ask whether this coefficient the octanol-water partition coefficient. is constant for a given SOC. Values of Kd Basically they put water and octanol for two polyaromatic hydrocarbons in (an eight-carbon alcohol) into a vessel, various soils are shown in Figure 2.26. added the organic compound of inter- For pyrene (which consists of four ben- est, and shook the combination up. zene rings stuck together), the Kd ratios After a period of rest, the water and oc- vary from about 300 to 1500. For phenanthrene (which consists of three Table 2.7: Solubility of six-carbon compounds. benzene rings stuck together), Kd varies from about 10 to 300. Clearly K is not a Compound Solubility d constant value for either compound. Hexane 10 mg/L But, K does appear to bear a relation to Hexanol 5,900 mg/L d the fraction of organic carbon in the var- Cyclohexane 55 mg/L ious sediments. What appears to be con- Benzene 1,780 mg/L stant is not Kd itself, but the ratio of Kd Phenol 82,000 mg/L to the fraction of organic carbon in the Chlorobenzene 448 mg/L sediment. This ratio is referred to as Koc:

2–40 Chapter 2: Stream Corridor Processes, Characteristics, and Functions 107 2,4,5,2',4',5' - PCB leptophos DDT 2,4,5,2',5' - PCB 106 DDE 4,4' - PCB dichlofenthion chlorpyrifos 105 ronnel dialifor methyl chlorpyrifos phosatone diphenyl ether 4 10 parathion dicapthon naphthalene p-dichlorobenzene fenitrothion iodobenzene 103 bromobenzene malathion chlorobenzene phosmel 2,4-D toluene carbon tetrachloride tetrachloroethylene salicylic acid 2 benzene n - Octanol: Water Partition Coefficient n - Octanol: Water 10 flourobenzene chloroform nitrobenzene benzoic acid phenylacetic acid phenoxyzcetic acid 0 10-3 10-2 10-1 1 10 102 103 104 105 106 Solubility in Water (µmoles/L)

Figure 2.24: Relationship between octanol/H2O partition coefficient and aqueous solubility. The relative solubility in water is a substance’s “Water Partition Coefficient.”

5 1800 600 slope = K 2,4,2', 4'- PCB 1500 oc 500 4 hexachloro- benzene 1200 400 3 biphenyl P-dichloro- 900 300 benzene diphenylether Pyrene pyrene d

2 K Phenanthrene Log BCF in Trout Muscle Log BCF in Trout

tetrachloroethylene 600 200 d carbontetrachloride K 1 300 100 2 3 4 5 6 7 phenanthrene Log Poct 0 0.0 .005 .010 .015 .020 .025 Figure 2.25: Relationship between octanol/ Fraction Organic Carbon water partition (Poct ) coefficient and bioaccu- mulation factor (BCF) in trout muscle. Water Figure 2.26: Relationship between pyrene, quality can be inferred by the accumulation phenanthrene, and fraction organic carbon. of contaminants in fish tissue. Contaminant concentrations in sediment vs.

water (Kd) are related to the amount of organ- ic carbon available.

Physical and Chemical Characteristics 2–41 Koc =Kd / fraction of organic carbon coefficient, the Henry’s Law constant in sediment (H), has been defined as the ratio of the concentration of an SOC in air in Various workers have related Koc to Kow and to water solubility (Table 2.8). equilibrium with its concentration in water:

Using Kow, Koc, and Kd to describe the partitioning of an SOC between water H = SOC concentration in air / and sediment has shown some utility, SOC concentration in water but this approach is not applicable to “SOC” = synthetic organic compounds the sorption of all organic molecules in A Henry’s Law constant for an SOC can all systems. Sorption of some SOC be estimated from the ratio of the com- occurs by hydrogen bonding, such as pound’s vapor pressure to its water sol- occurs in cation exchange or metal ubility. Organic compounds that are sorption to sediments (Figure 2.27). inherently volatile (generally low mole- Sorption is not always reversible; or at cular weight solvents) have very high least after sorption occurs, desorption Henry’s Law constants. But even com- may be very slow. pounds with very low vapor pressure Volatilization can partition into the atmosphere. DDT Organic compounds partition from and PCBs for example, have modest water into air by the process of Henry’s Law constants because their sol- volatilization. An air-water distribution ubility in water is so low. These SOC also have high Kd values and so may be-

Table 2.8: Regression equations for sediment adsorption coefficients (Koc ) for various contaminants.

Equationa No.b r2c Chemical Classes Represented

log Koc = -0.55 log S + 3.64 (S in mg/L) 106 0.71 Wide variety, mostly pesticides

log Koc = -0.54 log S + 0.44 10 0.94 Mostly aromatic or polynuclear aromatics; (S in mole fraction) two chlorinated

log Koc = -0.557 log S + 4.277 15 0.99 Chlorinated hydrocarbons (S in µ moles/L)d

log Koc = 0.544 log Kow + 1.377 45 0.74 Wide variety, mostly pesticides

log Koc = 0.937 log Kow - 0.006 19 0.95 Aromatics, polynuclear aromatics, triazines, and dinitroaniline herbicides

log Koc = 1.00 log Kow - 0.21 10 1.00 Mostly aromatic or polynuclear aromatics; two chlorinated

log Koc = 0.95 log Kow + 0.02 9 e S-triazines and dinitroaniline herbicides

log Koc = 1.029 log Kow - 0.18 13 0.91 Variety of insecticides, herbicides, and fungicides d log Koc = 0.524 log Kow + 0.855 30 0.84 Substituted phenylureas and alkyl-N-phenylcarbamates d,f log Koc = 0.0067 (p - 45N) + 0.237 29 0.69 Aromatic compounds, urea, 1.3.5-triazines, carbamates, and uracils

log Koc = 0.681 log 8CF(f) + 1.963 13 0.76 Wide variety, mostly pesticides

log Koc = 0.681 log 8CF(t) + 1.886 22 0.83 Wide variety, mostly pesticides

a Koc = soil (or sediment) adsorption coefficient; S = water solubility; Kow = octanol-water partition coefficient; BCF(f) = bioconcentration factor from flowing-water tests; BCF(t) = bioconcentration factor from model ecosystems; P = parachor; N = number of sites in molecule which can participate in the formation of a hydrogen bond. b No. = number of chemicals used to obtain regression equation. c r2 = correlation coefficient for regression equation. d Equation originally given in terms of Kom. The relationship Kom = Koc/1.724 was used to rewrite the equation in terms of Koc. e Not available. f Specific chemicals used to obtain regression equation not specified.

2–42 Chapter 2: Stream Corridor Processes, Characteristics, and Functions come airborne in association with par- ticulate matter. silica alumina Degradation O R O Si Al O H O O SOC can be transformed into a variety N H O Si C O O of degradation products. These degrada- Al O Si Al tion products may themselves degrade. O Ultimate degradation, or mineraliza- O adsorbents H O H tion, results in the oxidation of organic H carbon to carbon dioxide. Major trans- O2C C H H formation processes include photolysis, C hydrolysis, and oxidation-reduction re- Organic Bases Organic Acids actions. The latter are commonly medi- ated by biological systems. Figure 2.27: Two important types of hydrogen bonding involving natural organic matter and Photolysis refers to the destruction of a mineral surfaces. Some contaminants are car- compound by the energy of light. The ried by sediment particles that are sorbed onto energy of light varies inversely with its their surfaces by chemical bonding. wavelength (Figure 2.28). Long-wave light lacks sufficient energy to break chemical bonds. Short wave light (x-rays and gamma rays) is very destructive; fortunately for life on earth, this type of radiation largely is removed by our upper atmosphere. Light near the visi- ble spectrum reaches the earth’s surface and can break many of the bonds com- mon in SOC. The fate of organic sol- Figure 2.28: Energy of electromagnetic radia- vents following volatilization is usually tion compared with some selected bond ener- gies. Light breaks chemical bonds of some photolysis in the earth’s atmosphere. compounds through photolysis. Photolysis also can be important in the degradation of SOC in stream water. Wavelength Kilocalories Dissociation (nanometers) per Gram • Mole Energies for Hydrolysis refers to the splitting of an or- of Quanta Diatomic Molecules ganic molecule by water. Essentially 20 water enters a polar location on a mole- Infrared cule and inserts itself, with an H+ going 30 800 I • I to one part of the parent molecule and 40 Br • Br an OH- going to the other. The two 600 50 parts then separate. A group of SOC Visible Light 500 60 Cl • Cl called esters are particularly vulnerable C • S to degradation by hydrolysis. Many es- 400 70 C • N Near 80 C • Cl ters have been produced as pesticides Ultraviolet 350 C • O H • Br or plasticizers. 90 Middle 300 Oxidation-reduction reactions are what Ultraviolet S • S 100 H • Cl H • H fuels most metabolism in the bios- 110 250 phere. SOC are generally considered as Far C • F 120 O • O sources of reduced carbon. In such situ- Ultraviolet ations, what is needed for degradation 130 is a metabolic system with the appro- 140 200

Physical and Chemical Characteristics 2–43 priate enzymes for the oxidation of the Chemical consequences are rarely the compound. A sufficient supply of other immediate goal of most restoration nutrients and a terminal electron accep- actions. Plans that alter chemical tor are also required. processes and attributes are usually The principle of microbial infallibility in- focused on changing the physical and formally refers to the idea that given biological characteristics that are vital a supply of potential food, microbial to the restoration goals. communities will develop the meta- Toxic Concentrations of bolic capability to use that food for Bioavailable Metals biochemical energy. Not all degrada- tion reactions, however, involve the A variety of naturally occurring metals, oxidation of SOC. Some of the most ranging from arsenic to zinc, have been problematic organic contaminants established to be toxic to various forms are chlorinated compounds. of aquatic life when present in suffi- cient concentrations. The primary Chlorinated SOC do not exist naturally, mechanisms for water column toxicity so microbial systems generally are not of most metals is adsorption at the adapted for their degradation. Chlorine surface. While some studies indicate is an extremely electronegative element. that particulate metals may contribute The electronegativity of chlorine refers to toxicity, perhaps because of factors to its penchant for sucking on electrons. such as desorption at the gill surface, This tendency explains why chloride ex- the dissolved metal concentration most ists as an anion and why an attached closely approximates the fraction of chloride diminishes the solubility of metal in the water column that is an aromatic ring. Given this character, bioavailable. Accordingly, current EPA it is difficult for biological systems to policy is that dissolved metal concentra- oxidize chlorinated compounds. An tions should be used to set and mea- initial step in that degradation, there- sure compliance with water quality fore, is often reductive dechlorination. standards (40 CFR 22228-22236, May The chlorine is removed by reducing 4, 1995). For most metals, the dissolved the compound (i.e., by giving it elec- fraction is equivalent to the inorganic trons). After the chlorines are removed, ionic fraction. For certain metals, most degradation may proceed along oxida- notably mercury, the dissolved fraction tive pathways. The degradation of also may include the metal complexed chlorinated SOC thus may require a with organic binding agents (e.g., sequence of reducing and oxidizing methyl mercury, which can be produced environments, which water may experi- in sediments by methanogenic bacteria, ence as it moves between stream and is soluble and highly toxic, and can ac- hyporheic zones. cumulate through the food chain). The overall degradation of SOC often follows complex pathways. Figure 2.29 Toxic Concentrations of Bioavailable shows a complex web of metabolic Metals Across the Stream Corridor reaction for a single parent pesticide. Unlike synthetic organic compounds, Hydrolysis, reduction, and oxidation toxic metals are naturally occurring. In are all involved in the degradation of common with synthetic organics, met- SOC, and the distribution and behavior als may be loaded to waterbodies from of degradation products can be ex- both point and nonpoint sources. Pol- tremely variable in space and time. lutants such as copper, zinc, and lead

2–44 Chapter 2: Stream Corridor Processes, Characteristics, and Functions are often of concern in effluent from S wastewater treatment plants but are required under the NPDES program to HO POH meet numeric water quality standards. OH S O

Many of the toxic metals are present at HO P OEt HO P OEt H3PO4 oxidation significant concentrations in most soils OH OH but in sorbed nonbioavailable forms.

Sediment often introduces significant S S O S O concentrations of metals such as zinc HO P OEt OtE P OEt OtE P OEt into waterbodies. It is then a matter of OtE P OEt OtE P OEt O hydrolysis O O whether instream conditions promote OH OH bioavailable dissolved forms of the metal. hydrolysis hydrolysis oxidation

NO2 NO2 NO2 uv OH Nonpoint sources of metals first reflect parathion paraoxon the characteristics of watershed soils. In reduction reduction addition, many older industrial areas O O OtE O OEt OtE P OEt have soil concentrations of certain met- S NO2 O als that are elevated due to past indus- O S p- nitorphenol OtE P OEt OtE P OEt trial practices. Movement of metals from reduction soil to watershed is largely a function of O O the erosion and delivery of sediment. NO2 NO2 hydrolysis + In certain watersheds, a major source of OH NH2 NH2 hydrolysis metals loading is provided by acid mine O hydrolysis drainage. High acidity increases the sol- OtE P OEt ubility of many metals, and mines tend O NH2 to be in mineral-rich areas. Abandoned S p- aminophenol O mines are therefore a continuing source OtE P OEt OtE P OEt of toxic metals loading in many streams. hydrolysis OH OH NO 2 O Toxic Concentrations of Bioavailable hydrolysis inorganic Metals Along the Stream Corridor phosphate OtE POH Most metals have a tendency to leave OH the dissolved phase and attach to sus- pended particulate matter or form in- Figure 2.29: Metabolic reactions for a single parent pesticide. Particles break down through soluble precipitates. Conditions that processes of hydrolysis, oxidation, reduction, partition metals into particulate forms and photolysis. (presence of suspended sediments, dis- solved and particulate organic carbon, carbonates, bicarbonates, and other tral pH’s than in acidic or highly alka- ions that complex metals) reduce po- line waters. tential bioavailability of metals. Also, Ecological Functions of Soils calcium reduces metal uptake, appar- ently by competing with metals for ac- Soil is a living and dynamic resource tive uptake sites on gill membranes. pH that supports life. It consists of inor- is also an important water quality factor ganic mineral particles of differing sizes in metal bioavailability. In general, (clay, silt, and sand), organic matter in metal solubilities are lower at near neu- various stages of decomposition, nu- merous species of living organisms,

Physical and Chemical Characteristics 2–45 various water soluble ions, and various important to recognize these alterations gases and water. These components and to consider their impacts on goals. each have their own physical and chem- Soils perform vital functions through- ical characteristics which can either sup- out the landscape. One of the most im- port or restrict a particular form of life. portant functions of soil is to provide a Soils can be mineral or organic depend- physical, chemical, and biological set- ing on which material makes up the ting for living organisms. Soils support greater percentage in the soil matrix. biological activity and diversity for Mineral soils develop in materials plant and animal productivity. Soils weathered from rocks while organic also regulate and partition the flow of soils develop in decayed vegetation. water and the storage and cycling of nu- Both soils typically develop horizons or trients and other elements in the land- layers that are approximately parallel to scape. They filter, buffer, degrade, the soil surface. The extreme variety of immobilize, and detoxify organic and specific niches or conditions soil can inorganic materials and provide the me- create has enabled a large variety of chanical support living organisms need. fauna and flora to evolve and live under These hydrologic, geomorphic, and bio- those conditions. logic functions involve processes that Soils, particularly riparian and wetland help build and sustain stream corridors. soils, contain and support a very high Soil Microbiology diversity of flora and fauna both above and below the soil surface. A large vari- Organic matter provides the main source ety of specialized organisms can be of energy for soil microorganisms. Soil found below the soil surface, outnum- organic matter normally makes up 1 to bering those above ground by several or- 5 percent of the total weight in a min- ders of magnitude. Generally, organisms eral topsoil. It consists of original tissue, seen above ground are higher forms of partially decomposed tissue, and humus. life such as plants and wildlife. However, Soil organisms consume roots and vege- at and below ground, the vast majority tative detritus for energy and to build of life consists of plant roots having the tissue. As the original organic matter is responsibility of supporting the above decomposed and modified by microor- ground portion of the plant; many in- ganisms, a gelatinous, more resistant sects, mollusks, and fungi living on dead compound is formed. This material is organic matter; and an infinite number called humus. It is generally black or of bacteria which can live on a wide va- brown in color and exists as a colloid, a riety of energy sources found in soil. group of small, insoluble particles sus- pended in a gel. Small amounts of It is important to identify soil bound- humus greatly increase a soil’s ability to aries and to understand the differences hold water and nutrient ions which en- in soil properties and functions occur- hances plant production. Humus is an ring within a stream corridor in order indicator of a large and viable popula- to identify opportunities and limita- tion of microorganisms in the soil and it tions for restoration. Floodplain and increases the options available for vege- terrace soils are often areas of dense tative restoration. population and intensive agricultural development due to their flat slopes, Bacteria play vital roles in the organic proximity to water, and natural fertility. transactions that support plant growth. When planning stream corridor restora- They are responsible for three essential tion initiatives in developed areas, it is transformations: denitrification, sulfur

2–46 Chapter 2: Stream Corridor Processes, Characteristics, and Functions oxidation, and nitrogen fixation. Micro- slope occurs on the floodplain side bial reduction of nitrate to nitrite and of the natural , so the floodplain then to gaseous forms of nitrogen is becomes lowest at a point far from termed denitrification. A water content the river. Parent materials decrease in of 60 percent generally limits denitrifi- grain size away from the river due to cation and the process only occurs at the decrease in sediment-transport soil temperatures between 5°C and capacity in the slackwater areas. 75°C. Other soil properties optimizing Soils of topographic floodplains. Slightly the rate of denitrification include a pH higher areas within and outside the between 6 and 8, soil aeration below active floodplain are defined as the the biological oxygen demand of the or- topographic floodplain. They are ganisms in the soil, sufficient amounts usually inundated less frequently of water-soluble carbon compounds, than the active floodplain, so soils readily available nitrate in the soil, and may exhibit more profile develop- the presence of enzymes needed to start ment than the younger soils on the the reaction. active floodplain.

Landscape and Topographic Soils of terraces. Abandoned flood- Position plains, or terraces, are the next high- est surfaces in stream corridors. These Soil properties change with topographic surfaces rarely flood. Terrace soils, in position. Elevation differences generally general, are coarser textured than mark the boundaries of soils and floodplain soils, are more freely drainage conditions in stream corridors. drained, and are separated from Different landforms generally have dif- stream processes. ferent types of sediment underlying them. Surface and subsurface drainage Upon close examination, floodplain patterns also vary with landforms. deposits can reveal historical events of given watersheds. Soil profile develop- Soils of active channels. The active ment offers clues to the recent and geo- channel forms the lowest and usually logic history at a site. Intricate and youngest surfaces in the stream corri- complex analysis methods such as car- dor. There is generally no soil devel- bon dating, pollen analysis, ratios of oped on these surfaces since the certain isotopes, etc. can be used to unconsolidated materials forming piece together an area’s history. Cycles the stream bottom and banks are of erosion or deposition can at times be constantly being eroded, transported, linked to catastrophic events like forest and redeposited. fires or periods of high or low precipita- Soils of active floodplains. The next tion. Historical impacts of civilization, highest surface in the stream corridor such as extensive agriculture or denuda- is the flat, depositional surface of the tion of forest cover will at times also active floodplain. This surface floods leave identifiable evidence in soils. frequently, every 2 out of 3 years, so it receives sediment deposition. Soil Temperature and Moisture Relationships Soils of natural . Natural levees are built adjacent to the stream by Soil temperature and moisture control deposition of coarser, suspended sed- biological processes occurring in soil. iment dropping out of overbank Average and expected precipitation and flows during floods. A gentle back- temperature extremes are critical pieces

Physical and Chemical Characteristics 2–47 of information when considering goals present in wetlands areas, creating such for restoration initiatives. The mean an- drastic changes in physical and chemical nual soil temperature is usually very conditions that most species found in similar to the mean annual air tempera- uplands cannot survive. Hence the com- ture. Soil temperatures do experience position of flora and fauna in wetlands daily, seasonal, and annual fluctuations are vastly different and unique, espe- caused by solar radiation, weather pat- cially in wetlands subject to permanent terns, and climate. Soil temperatures are or prolonged saturation or flooding. also affected by aspect, latitude, and ele- Hydric soils are defined as those that are vation. saturated, flooded, or ponded long Soil moisture conditions change sea- enough during the growing season to sonally. If changes in vegetation species develop anaerobic conditions in the and composition are being considered upper part. These anaerobic conditions as part of a restoration initiative, a affect the reproduction, growth, and graph comparing monthly precipitation survival of plants. The driving process and evapotranspiration for the vegeta- behind the formation of hydric soils is tion should be constructed. If the water flooding and/or soil saturation near the table and capillary fringe is below the surface for prolonged periods (usually predicted rooting depth, and the graph more the seven days) during the grow- indicates a deficit in available water, ir- ing season (Tiner and Veneman 1989). rigation may be required. If no supple- The following focuses primarily on mental water is available, different plant mineral hydric soil properties, but or- species must be considered. ganic soils such as peat and muck may The soil moisture gradient can decrease be present in the stream corridor. from 100 percent to almost zero along In aerated soil environments, atmos- the transriparian continuum as one pheric oxygen enters surface soils progresses from the stream bottom, through gas diffusion, as soil pores are across the riparian zone, and into the mostly filled with air. Aerated soils are higher elevations of the adjacent up- found in well drained uplands, and gen- lands (Johnson and Lowe 1985), which erally all areas having a water table well results in vast differences in moisture below the root zone. In saturated soils, available to vegetation. This gradient in pores are filled with water, which diffuse soil moisture directly influences the gases very slowly compared to the at- characteristics of the ecological commu- mosphere. Only small amounts of oxy- nities of the riparian, transitional, and gen can dissolve in soil moisture, which upland zones. These ecological differ- then disperses into the top few inches of ences result in the presence of two eco- soil. Here, soil microbes quickly deplete tones along the stream corridor—an all available free oxygen in oxidizing or- aquatic-wetland/riparian ecotone and a ganic residue to carbon dioxide. This re- non-wetland riparian/floodplain eco- action produces an anaerobic tone—which increase the edge effect of chemically reducing environment in the riparian zone and, therefore, the bi- which oxidized compounds are changed ological diversity of the region. to reduced compounds that are soluble Wetland Soils and also toxic to many plants. The rate of diffusion is so slow that oxygenated Wet or “hydric” soils present special conditions cannot be reestablished challenges to plant life. Hydric soils are under such circumstances. Similar mi-

2–48 Chapter 2: Stream Corridor Processes, Characteristics, and Functions crobial reactions involving decomposi- ture encountered with depth are related tion of organic matter in waterlogged to stratification of sediments sorted by anaerobic environments produce ethyl- size during deposition by flowing water. ene gas, which is highly toxic to plant Clay formation tends to occur in place roots and has an even stronger effect and little translocation happens within than a lack of oxygen. After all free oxy- the profile, as essentially no water gen is utilized, anaerobic microbes re- moves through the soil to transport the duce other chemical constituents of the particles. Due to the reactivity of wet soil including nitrates, manganese ox- soils, clay formation tends to progress ides, and iron oxides, creating a further much faster than in uplands. reduced condition in the soil. Soils which are seasonally saturated or Prolonged anaerobic reducing condi- have a fluctuating water table result in tions result in the formation of readily distinct horizonation within the profile. visible signs of reduction. The typical As water regularly drains through the gray colors encountered in wet soils are profile, it translocates particles and the result of reduced iron, and are transports soluble ions from one layer known as gleyed soils. After iron oxides to another, or entirely out of the profile. are depleted, sulfates are reduced to sul- Often, these soils have a thick horizon fides, producing the rotten egg odor of near the surface which is stripped of all wet soils. Under extremely waterlogged soluble materials including iron; known conditions, carbon dioxide can be re- as a depleted matrix. Seasonally saturated duced to methane. Methane gas, also soils usually have substantial organic known as “swamp gas” can be seen at matter accumulated at the surface, night, as it fluoresces. nearly black in color. The organics add Some wetland plants have evolved spe- to the cation exchange capacity of the cial mechanisms to compensate for hav- soil, but base saturation is low due to ing their roots immersed in anoxic stripping and overabundance of hydro- environments. Water lilies, for example, gen ions. During non-saturated times, force a gas exchange within the entire organic materials are exposed to atmos- plant by closing their stomata during pheric oxygen, and aerobic decomposi- the heat of the day to raise the air pres- tion can take place which results in sure within special conductive tissue massive liberation of hydrogen ions. (aerenchyma). This process tends to in- Seasonally wet soils also do not retain troduce atmospheric oxygen deep into base metals well, and can release high the root crown, keeping vital tissues concentrations of metals in wet cycles alive. Most emergent wetland plants following dry periods. simply keep their root systems close to Wet soil indicators will often remain in the soil surface to avoid anaerobic con- the soil profile for long periods of time ditions in deeper strata. This is true of (even after drainage), revealing the his- sedges and rushes, for example. torical conditions which prevailed. Ex- When soils are continually saturated amples of such indicators are rust throughout, reactions can occur equally colored iron deposits which at one time throughout the soil profile as opposed were translocated by water in reduced to wet soils where the water level fluctu- form. Organic carbon distribution from ates. This produces soils with little past fluvial deposition cycles or zones zonation, and materials tend to be of stripped soils resulting from wetland more uniform. Most differences in tex- situations are characteristics which are extremely long lived.

Physical and Chemical Characteristics 2–49 Summary Restoration activities may interact in a variety of complex ways with water quality, affecting both This section provides only a brief overview of the the delivery and impact of water quality stres- diverse and complex chemistry; nevertheless, two sors. key points should be evident to restoration practi- tioners: Table 2.9 shows how a sample selection of com- mon stream restoration and watershed manage- Restoring physical habitat cannot restore biologi- ment practices may interact with the water quality cal integrity of a system if there are water quality parameters described in this section. constraints on the ecosystem.

Table 2.9: Potential water quality impacts of selected stream restoration and watershed management practices.

Restoration Fine Water Salinity pH Dissolved Nutrients Toxics Activities Sediment Temperature Oxygen Loads Reduction of Decrease Decrease Decrease Increase/ Increase Decrease Decrease land-disturbing decrease activities

Limit impervious Decrease Decrease Negligible Increase Increase Decrease Decrease surface area in effect the watershed

Restore riparian Decrease Decrease Decrease Decrease Increase Decrease Decrease vegetation Restore Decrease Increase/ Increase/ Increase/ Decrease Increase Increase wetlands decrease decrease decrease Stabilize channel Decrease Decrease Decrease Decrease Increase Decrease Negligible and restore effect under-cut banks

Create drop Increase Negligible Negligible Increase/ Increase Negligible Decrease structures effect effect decrease effect Reestablish Negligible Negligible Negligible Increase/ Increase Negligible Negligible riffle substrate effect effect effect decrease effect effect

2–50 Chapter 2: Stream Corridor Processes, Characteristics, and Functions 2.D Biological Community Characteristics

Successful stream restoration is based Terrestrial Vegetation REVERSE on an understanding of the relation- ships among physical, chemical, and bi- The ecological integrity of stream corri- ological processes at varying time scales. dor ecosystems is directly related to the Often, human activities have acceler- integrity and ecological characteristics ated the temporal progression of these of the plant communities that make up Review Section processes, resulting in unstable flow and surround the corridor. These plant C for further discussion of patterns and altered biological structure communities are a valuable source of the ecological and function of stream corridors. This energy for the biological communities, provide physical habitat, and moderate functions of section discusses the biological struc- soils. ture and functions of stream corridors solar energy fluxes to and from the sur- in relation to geomorphologic, hydro- rounding aquatic and terrestrial ecosys- logic, and water quality processes. The tems. Given adequate moisture, light, interrelations between the watershed and temperature, the vegetative com- and the stream, as well as the cause and munity grows in an annual cycle of ac- effects of disturbances to these interrela- tive growth/production, senescence, and tionships are also discussed. Indices relative dormancy. The growth period is and approaches for evaluating stream subsidized by incidental solar radiation, corridor functions are provided in which drives the photosynthetic process Chapter 7. through which inorganic carbon is con- verted to organic plant materials. A por- Terrestrial Ecosystems tion of this organic material is stored as above- and below-ground biomass, The biological community of a stream while a significant fraction of organic corridor is determined by the character- matter is lost annually via senescence, istics of both terrestrial and aquatic fractionation, and leaching to the or- ecosystems. Accordingly, the discussion ganic soil layer in the form of leaves, of biological communities in stream twigs, and decaying roots. This organic corridors begins with a review of terres- fraction, rich in biological activity of trial ecosystems. microbial flora and microfauna, repre- Ecological Role of Soil sents a major storage and cycling pool of available carbon, nitrogen, phospho- Terrestrial ecosystems are fundamen- rus, and other nutrients. tally tied to processes within the soil. The distribution and characteristics of The ability of a soil to store and cycle vegetative communities are determined nutrients and other elements depends by climate, water availability, topo- on the properties and microclimate graphic features, and the chemical and (i.e., moisture and temperature) of the physical properties of the soil, including soil, and the soil’s community of organ- moisture and nutrient content. The isms (Table 2.10). These factors also de- characteristics of the plant communities termine its effectiveness at filtering, directly influence the diversity and in- buffering, degrading, immobilizing, and tegrity of the faunal communities. Plant detoxifying other organic and inorganic communities that cover a large area and materials. that are diverse in their vertical and hor- izontal structural characteristics can support far more diverse faunal com-

Biological Community Characteristics 2–51 Animals these communities reflect the recent Macro Subsisting largely on plant materials historical (100 years or less) physical Small mammals—squirrels, gophers, woodchucks, mice, shrews conditions of the landscape. Insects—springtails, ants, beetles, grubs, etc. The quantity of terrestrial vegetation, as Millipedes well as its species composition, can di- Sowbugs (woodlice) rectly affect stream channel characteris- Mites tics. Root systems in the streambank Slugs and snails can bind bank sediments and moderate Earthworms erosion processes. Trees and smaller Largely predatory woody debris that fall into the stream can deflect flows and induce erosion at Moles some points and deposition at others. Insects—many ants, beetles, etc. Thus woody debris accumulation can Mites, in some cases influence pool distribution, organic Centipedes matter and nutrient retention, and the Spiders formation of microhabitats that are im- Micro Predatory or parasitic or subsisting on plant residues portant fish and invertebrate aquatic Nematodes communities. Protozoa Streamflow also can be affected by the Rotifers abundance and distribution of terres- trial vegetation. The short-term effects Plants of removing vegetation can result in an Roots of higher plants immediate short-term rise in the local Algae water table due to decreased evapotran- Green spiration and additional water entering Blue-green the stream. Over the longer term, how- Diatoms ever, after removal of vegetation, the Fungi baseflow of streams can decrease and Mushroom fungi water temperatures can rise, particularly Yeasts in low-order streams. Also, removal of Molds vegetation can cause changes in soil Actinomycetes of many kinds temperature and structure, resulting in Bacteria decreased movement of water into and through the soil profile. The loss of sur- Aerobic Autotrophic face litter and the gradual loss of or- Heterotrophic ganic matter in the soil also contribute Anaerobic Autotrophic to increased surface runoff and de- Heterotrophic creased infiltration.

Table 2.10: Groups of organisms commonly In most instances, the functions of veg- present in soils. etation that are most apparent are those that influence fish and wildlife. At the munities than relatively homogenous landscape level, the fragmentation of plant communities, such as meadows. native cover types has been shown to As a result of the complex spatial and significantly influence wildlife, often fa- temporal relationships that exist be- voring opportunistic species over those tween floral and faunal communities, requiring large blocks of contiguous current ecological characteristics of habitat. In some systems, relatively

2–52 Chapter 2: Stream Corridor Processes, Characteristics, and Functions small breaks in corridor continuity can floodplain, and the influx of turbid and have significant impacts on animal cooler channel water influences light movement or on the suitability of penetration and temperature of the stream conditions to support certain inundated floodplain. aquatic species. In others, establishing Stream Corridor Scale corridors that are structurally different from native systems or that are inappro- At the stream corridor scale, the compo- priately configured can be equally dis- sition and regeneration patterns of veg- ruptive. Narrow corridors that are etation are characterized in terms of essentially edge habitat may encourage horizontal complexity. Floodplains along generalist species, nest parasites, and unconstrained channels typically are predators, and, where corridors have vegetated with a mosaic of plant com- been established across historic barriers munities, the composition of which to animal movement, they can disrupt varies in response to available surface the integrity of regional animal assem- and ground water, differential patterns blages (Knopf et al. 1988). of flooding, fire, and predominant winds, sediment deposition, and oppor- Landscape Scale tunities for establishing vegetation. The ecological characteristics and distri- A broad floodplain of the southern, bution of plant communities in a wa- midwestern, or eastern United States tershed influence the movement of may support dozens of relatively dis- water, sediment, nutrients, and wildlife. tinct forest communities in a complex Stream corridors provide links with mosaic reflecting subtle differences in other features of the landscape. Links soil type and flood characteristics (e.g., may involve continuous corridors be- frequency, depth, and duration). In tween headwater and valley floor contrast, while certain western stream ecosystems or periodic interactions be- systems may support only a few woody tween terrestrial systems. Wildlife use species, these systems may be struc- corridors to disperse juveniles, to mi- turally complex due to constant rework- grate, and to move between portions of ing of substrates by the stream, which their home range. Corridors of a natural produces a mosaic of stands of varying origin are preferred and include streams ages. The presence of side channels, and rivers, riparian strips, mountain oxbow lakes, and other topographic passes, isthmuses, and narrow straits variation can be viewed as elements of (Payne and Bryant 1995). structural variation at the stream corri- It is important to understand the differ- dor level. Riparian areas along con- ences between a stream-riparian ecosys- strained stream channels may consist tem and a river-floodplain ecosystem. primarily of upland vegetation orga- Flooding in the stream-riparian ecosys- nized by processes largely unrelated to tem is brief and unpredictable. The ri- stream characteristics, but these areas parian zone supplies nutrients, water, may have considerable influence on the and sediment to the stream channel, stream ecosystem. and riparian vegetation regulates tem- The , as dis- perature and light. In the river-flood- cussed in Chapter 1, is also generally plain ecosystem, floods are often more applicable to the vegetative components predictable and longer lasting, the river of the riparian corridor. Riparian vegeta- channel is the donor of water, sedi- tion demonstrates both a transriparian ment, and inorganic nutrients to the gradient (across the valley) and an

Biological Community Characteristics 2–53 intra-riparian (longitudinal, eleva- Plant Communities tional) gradient (Johnson and Lowe The sensitivity of animal communities 1985). In the west, growth of riparian to vegetative characteristics is well rec- vegetation is increased by the “ ognized. Numerous animal species are effect” resulting when cool moist air associated with particular plant com- spills downslope from higher elevations munities, many require particular devel- (Figure 2.30). This cooler air settles in opmental stages of those communities and creates a more moist mi- (e.g., old-growth), and some depend on crohabitat than occurs on the surround- particular habitat elements within those ing slopes. These canyons also serve as communities (e.g., snags). The structure water courses. The combination of of streamside plant communities also moist, cooler edaphic and atmospheric directly affects aquatic organisms by conditions is conducive to plant and providing inputs of appropriate organic animal species at lower than normal al- materials to the aquatic food web, by titudes, often in disjunct populations or shading the water surface and providing in regions where they would not other- cover along banks, and by influencing wise occur (Lowe and Shannon 1954). instream habitat structure through in-

can yon eff ect —d ow nh ill alder-walnut dr ai na ge of co ol , m oi st a sycamore-ash ir

pla nt a n d a nimal cottonwood- dispersal willow

channel

floodplain

stream corridor

Figure 2.30: Canyon effect. Cool moist air settles in canyons and creates microhabitat that occurs on surrounding slopes.

2–54 Chapter 2: Stream Corridor Processes, Characteristics, and Functions puts of woody debris (Gregory et al. Increasing the patch size (area) of a 1991). streamside vegetation type, increasing Plant communities can be viewed in the number of woody riparian tree size terms of their internal complexity (Fig- classes, and increasing the number of ure 2.31). Complexity may include the species and growth forms (herb, shrub, number of layers of vegetation and the tree) of native riparian-dependent vege- species comprising each layer; competi- tation can increase the number of tive interactions among species; and the guilds and the amount of forage, result- presence of detrital components, such ing in increased species richness and as litter, downed , and snags. Veg- biomass (numbers). Restoration tech- etation may contain tree, sapling, shrub niques can change the above factors. (subtree), vine, and herbaceous sub- The importance of horizontal complex- shrub (herb-grass-forb) layers. Microto- ity within stream corridors to certain pographic relief and the ability of water animal species also has been well estab- to locally pond also may be regarded as lished. The characteristic compositional, characteristic structural components. structural, and topographic complexity Vertical complexity, described in the con- of southern floodplain forests, for ex- cept of diversity of strata or foliage ample, provides the range of resources height diversity in ecological literature, and foraging conditions required by was important to studies of avian habi- many wintering waterfowl to meet par- tat by Carothers et al. (1974) along the ticular requirements of their life cycles Verde River, a fifth- or sixth-order at the appropriate times (Fredrickson stream in central Arizona. Findings 1978); similar complex relationships showed a high correlation between ri- have been reported for other vertebrates parian bird species diversity and foliage and invertebrates in floodplain habitats height diversity of riparian vegetation (Wharton et al. 1982). In parts of the (Carothers et al. 1974). Short (1985) arid West, the unique vegetation struc- demonstrated that more structurally di- ture in riparian systems contrasts dra- verse vegetative habitats support a greater number of guilds (groups of trees species with closely related niches in a community) and therefore a larger number of species. Species and age composition of vegeta- tion structure also can be extremely im- portant. Simple vegetative structure, such as an herbaceous layer without woody overstory or old woody riparian shrubs trees without smaller size classes, cre- ates fewer niches for guilds. The fewer guilds there are, the fewer species there are. The quality and vigor of the vegeta- herbaceous tion can affect the productivity of fruits, subshrubs seeds, shoots, roots, and other vegeta- tive material, which provide food for wildlife. Poorer vigor can result in less food and fewer consumers (wildlife). Figure 2.31: Vertical complexity. Complexity may include a number of layers of vegetation.

Biological Community Characteristics 2–55 matically with the surrounding uplands corridor and should take advantage of and provides essential habitat for many the successional process by planting animals (Knopf et al. 1988). Even hardy early-successional species to sta- within compositionally simple riparian bilize an eroding streambank, while systems, different developmental stages planning for the eventual replacement may provide different resources. of these species by longer-lived and Plant communities are distributed on higher-successional species. floodplains in relation to flood depth, Terrestrial Fauna duration, and frequency, as well as vari- ations in soils and drainage condition. Stream corridors are used by wildlife Some plant species, such as cottonwood more than any other habitat type (Populus sp.), willows (Salix sp.), and (Thomas et al. 1979) and are a major silver maple (Acer saccharinum), are source of water to wildlife populations, adapted to colonization of newly de- especially large mammals. For example, posited sediments and may require very 60 percent of Arizona’s wildlife species specific patterns of flood recession dur- depend on riparian areas for survival ing a brief period of seedfall to be suc- (Ohmart and Anderson 1986). In the cessfully established (Morris et al. 1978, Great Basin area of Utah and Nevada, Rood and Mahoney 1990). The resul- 288 of the 363 identified terrestrial ver- tant pattern is one of even-aged tree tebrate species depend on riparian stands established at different intervals zones (Thomas et al. 1979). Because of and locations within the active meander their wide suitability for upland and ri- belt of the stream. Other species, such parian species, midwestern stream corri- as the bald cypress (Taxodium dis- dors associated with prairie grasslands tichum), are particularly associated with support a wider diversity of wildlife oxbow lakes formed when streams cut than the associated uplands. Stream cor- off channel segments, while still others ridors play a large role in maintaining are associated with microtopographic biodiversity for all groups of vertebrates. variations within floodplains that re- The faunal composition of a stream cor- flect the slow migration of a stream ridor is a function of the interaction of channel across the landscape. food, water, cover, and spatial arrange- Plant communities are dynamic and ment (Thomas et al. 1979). These habi- change over time. The differing regener- tat components interact in multiple ation strategies of particular vegetation ways to provide eight habitat features of types lead to characteristic patterns of stream corridors: plant succession following disturbances Presence of permanent sources of in which pioneer species well-adapted water. to bare soil and plentiful light are grad- ually replaced by longer-lived species High primary productivity and bio- that can regenerate under more shaded mass. and protected conditions. New distur- Dramatic spatial and temporal con- bances reset the successional process. trasts in cover types and food avail- Within stream corridors, flooding, ability. channel migration, and, in certain bio- Critical microclimates. mes, fire, are usually the dominant nat- ural sources of disturbance. Restoration Horizontal and vertical habitat diver- practitioners should understand pat- sity. terns of natural succession in a stream

2–56 Chapter 2: Stream Corridor Processes, Characteristics, and Functions Maximized edge effect. Forested connectors between habitats establish continuity between forested Effective seasonal migration routes. uplands that may be surrounded by un- High connectivity between vegetated forested areas. These act as feeder lines patches. for dispersal and facilitate repopulation Stream corridors offer the optimal habi- by plants and animals. Thus, connectiv- tat for many forms of wildlife because ity is very important for retaining biodi- of the proximity to a water source and versity and genetic integrity on a an ecological community that consists landscape basis. primarily of hardwoods in many parts However, the linear distribution of of the country, which provide a source habitat, or edge effect, is not an effec- of food, such as nectar, catkins, buds, tive indicator of habitat quality for all fruit, and seeds (Harris 1984). Up- species. Studies in island biogeography, stream sources of water, nutrients, and using habitat islands rather than energy ultimately benefit downstream oceanic islands, demonstrate that a locations. In turn, the fish and wildlife larger habitat island supports both a return and disperse some of the nutri- larger population of birds and also a ents and energy to uplands and wet- larger number of species (Wilson and lands during their movements and Carothers 1979). Although a continu- migrations (Harris 1984). ous corridor is most desirable, the next Water is especially critical to fauna in preferable situation is minimal frag- areas such as the Southwest or Western mentation, i.e., large plots (“islands”) Prairie regions of the U.S. where stream of riparian vegetation with minimal corridors are the only naturally occur- spaces between the large plots. ring permanent sources of water on the Reptiles and Amphibians landscape. These relatively moist envi- ronments contribute to the high pri- Nearly all amphibians (salamanders, mary productivity and biomass of the toads, and frogs) depend on aquatic riparian area, which contrasts dramati- habitats for reproduction and overwin- cally with surrounding cover types and tering. While less restricted by the pres- food sources. In these areas, stream cor- ence of water, many reptiles are found ridors provide critical microclimates primarily in stream corridors and ripar- that ameliorate the temperature and ian habitats. Thirty-six of the 63 reptile moisture extremes of uplands by pro- and amphibian species found in west- viding water, shade, evapotranspiration, central Arizona were found to use ripar- and cover. ian zones. In the Great Basin, 11 of 22 reptile species require or prefer riparian The spatial distribution of vegetation is zones (Ohmart and Anderson 1986). also a critical factor for wildlife. The lin- ear arrangement of streams results in a Birds maximized edge effect that increases Birds are the most commonly observed species richness because a species can terrestrial wildlife in riparian corridors. simultaneously access more than one Nationally, over 250 species have been cover (or habitat) type and exploit the reported using riparian areas during resources of both (Leopold 1933). some part of the year. Edges occur along multiple habitat The highest known density of nesting types including the aquatic, riparian, birds in North America occurs in south- and upland habitats. western cottonwood habitats (Carothers

Biological Community Characteristics 2–57 and Johnson 1971). Seventy-three per- Riparian areas provide tall dense cover cent of the 166 breeding bird species in for roosts, water, and abundant prey for the Southwest prefer riparian habitats a number of bat species, including the (Johnson et al. 1977). little brown bat (Myotis lucifugus), big Bird species richness in midwestern brown bat (Eptesicus fuscus), and the stream corridors reflects the vegetative pallid bat (Antrozous pallidus). Brinson diversity and width of the corridor. et al. (1981) tabulated results from sev- Over half of these breeding birds are eral studies on mammals in riparian species that forage for insects on foliage areas of the continental U.S. They con- (vireos, warblers) or species that forage cluded that the number of mammal for seeds on the ground (doves, orioles, species generally ranges from five to 30, grosbeaks, sparrows). Next in abun- with communities including several dance are insectivorous species that for- furbearers, one or more large mammals, age on the ground or on trees and a few small to medium mammals. (thrushes, woodpeckers). Hoover and Wills (1984) reported 59 Smith (1977) reported that the distrib- species of mammals in cottonwood ri- ution of bird species in forested habi- parian woodlands of Colorado, second tats of the Southeast was closely linked only to pinyon-juniper among eight to soil moisture. Woodcock (Scolopax other forested cover types in the region. minor) and snipe (Gallinago gallinago), Fifty-two of the 68 mammal species red-shouldered hawks (Buteo lineatus), found in west-central Arizona in Bureau hooded and prothonotary warblers of Land Management inventories use ri- (Wilsonia citrina, Protonotaria citrea), parian habitats. Stamp and Ohmart and many other passerines in the (1979) and Cross (1985) found that ri- Southeast prefer the moist ground con- parian areas had a greater diversity and ditions found in riverside forests and biomass of small mammals than adja- shrublands for feeding. The cypress and cent upland areas. mangrove swamps along Florida’s wa- The contrast between the species diver- terways harbor many species found al- sity and productivity of mammals in most nowhere else in the Southeast. the riparian zone and that of the sur- rounding uplands is especially high in Mammals arid and semiarid regions. However, The combination of cover, water, and bottomland hardwoods in the eastern food resources in riparian areas make U.S. also have exceptionally high habi- them desirable habitat for large mam- tat values for many mammals. For ex- mals such as mule deer (Odocoileus ample, bottomland hardwoods support hemionus), white-tailed deer (Odocoileus white-tail deer populations roughly virginianus), moose (Alces alces), and elk twice as large as equivalent areas of up- (Cervus elaphus) that can use multiple land forest (Glasgow and Noble 1971). habitat types. Other mammals depend on riparian areas in some or all of their Stream corridors are themselves influ- range. These include otter (Lutra enced by certain animal activities (For- canadensis), ringtail (Bassarisdus astutus), man 1995). For example, beavers build raccoon (Procyon lotor), beaver (Castor dams that cause ponds to form within a canadensis), muskrat (Ondatra zibethi- stream channel or in the floodplain. The cus), swamp rabbit (Sylvilagus aquati- pond kills much of the existing vegeta- cus), short-tailed shrew (Blarina tion, although it does create wetlands brevicauda), and mink (Mustela vison). and open water areas for fish and mi-

2–58 Chapter 2: Stream Corridor Processes, Characteristics, and Functions gratory waterfowl. If appropriate woody tivities. A stream’s cross-sectional shape plants in the floodplain are scarce, and dimensions, its slope and confine- beavers extend their cutting activities ment, the grain-size distribution of bed into the uplands and can significantly sediments, and even its planform affect alter the riparian and stream corridors. aquatic habitat. Under less disturbed Over time, the pond is replaced by a situations, a narrow, steep-walled cross mudflat, which becomes a meadow and section provides less physical area for eventually gives way to woody succes- habitat than a wider cross section with sional stages. Beaver often then build a less steep sides, but may provide more dam at a new spot, and the cycle begins biologically rich habitat in deep pools anew with only a spatial displacement. compared to a wider, shallower stream The sequence of beaver dams along a corridor. A steep, confined stream is a stream corridor may have major effects high-energy environment that may limit on hydrology, sedimentation, and min- habitat occurrence, diversity, and stabil- eral nutrients (Forman 1995). Water ity. Many steep, fast flowing streams are from stormflow is held back, thereby af- coldwater salmonid streams of high fording some measure of . value. Unconfined systems flood fre- Silts and other fine sediments accumu- quently, which can promote riparian late in the pond rather than being habitat development. Habitat increases washed downstream. Wetland areas with stream sinuosity. Uniform sedi- usually form, and the water table rises ment size in a streambed provides less upstream of the dam. The ponds com- potential habitat diversity than a bed bine slow flow, near-constant water lev- with many grain sizes represented. els, and low turbidity that support fish Habitat subsystems occur at different and other aquatic organisms. Birds may scales within a stream system (Frissell use beaver ponds extensively. The wet- et al. 1986) (Figure 2.32). The grossest lands also have a relatively constant scale, the stream system itself, is mea- water table, unlike the typical fluctua- sured in thousands of feet, while seg- tions across a floodplain. Beavers cut- ments are measured in hundreds of feet ting trees diminish the abundance of and reaches are measured in tens of such species as elm (Ulmus spp.) and feet. A reach system includes combina- ash (Fraxinus spp.) but enhance the tions of debris dams, boulder cascades, abundance of rapidly sprouting species, rapids, step/pool sequences, pool/riffle such as alder (Alnus spp.), willow, and sequences, or other types of streambed poplar (Populus spp.). forms or “structures,” each of which could be 10 feet or less in scale. Fris- Aquatic Ecosystems sell’s smallest scale habitat subsystem includes features that are a foot or less Aquatic Habitat in size. Examples of these microhabitats The biological diversity and species include leaf or stick detritus, sand or silt abundance in streams depend on the over cobbles or other coarse material, diversity of available habitats. Naturally moss on boulders, or fine gravel functioning, stable stream systems pro- patches. mote the diversity and availability of Steep slopes often form a step/pool se- habitats. This is one of the primary rea- quence in streams, especially in cobble, sons stream stability and the restoration boulder, and bedrock streams. Each of natural functions are always consid- step acts as a miniature grade stabiliza- ered in stream corridor restoration ac- tion structure. The steps and pools work

Biological Community Characteristics 2–59 leaf and stick boulder detritus in cascade margin

sand-silt over cobbles

transverse bar over cobbles

moss on boulder

fine gravel debris dam patch

Stream Segment Segment System Reach System “Pool/Riffle” System Microhabitat System

Figure 2.32: Hierarchical organization of a stream system and its habitat subsystems. Approximate linear spatial scale, appropriate to second- or third-order mountain stream.

together to distribute the excess energy A wetland is an ecosystem that depends available in these steeply sloping sys- on constant or recurrent shallow inun- tems. They also add diversity to the dation or saturation at or near the sur- habitat available. Cobble- and gravel- face of the substrate. The minimum bottomed streams at less steep slopes essential characteristics of a wetland are form pool/riffle sequences, which also recurrent, sustained inundation or satu- increase habitat diversity. Pools provide ration at or near the surface and the space, cover, and nutrition to fish and presence of physical, chemical, and bio- they provide a place for fish to seek logical features that reflect recurrent shelter during storms, droughts, and sustained inundation or saturation. other catastrophic events. Upstream mi- Common diagnostic features of wet- gration of many salmonid species typi- lands are hydric soils and hydrophytic cally involves rapid movements through vegetation. These features will be pre- shallow areas, followed by periods of sent except where physicochemical, bi- rest in deeper pools (Spence et al. otic, or anthropogenic factors have 1996). removed them or prevented their devel- opment (National Academy of Sciences Wetlands 1995). Wetlands may occur in streams, Stream corridor restoration initiatives riparian areas, and floodplains of the may include restoration of wetlands stream corridor. The riparian area or such as riverine-type bottomland hard- zone may contain both wetlands and wood systems or riparian wetlands. non-wetlands. While wetland restoration is a specific Wetlands are transitional between terres- topic better addressed in other references trial and aquatic systems where the (e.g., Kentula et al. 1992), a general dis- water table is usually at or near the cussion of wetlands is provided here. surface or the land is covered by shallow Stream corridor restoration initiatives water (Cowardin et al. 1979). For vege- should be designed to protect or restore tated wetlands, water creates conditions the functions of associated wetlands. that favor the growth of hydrophytes— plants growing in water or on a sub-

2–60 Chapter 2: Stream Corridor Processes, Characteristics, and Functions strate that is at least periodically defi- cient in oxygen as a result of excessive water content (Cowardin et al. 1979) and promotes the development of hy- dric soils—soils that are saturated, The riparian zone is a classic example of the maximized flooded, or ponded long enough during value that occurs when two or more habitat types meet. the growing season to develop anaero- There is little question of the substantial value of riparian bic conditions in the upper part (Na- habitats in the United States. The Fish and Wildlife tional Academy of Sciences 1995). Service has developed protocols to classify and map Wetland functions include fish and riparian areas in the West in conjunction with the wildlife habitat, water storage, sediment National Wetlands Inventory (NWI). NWI will map ripari- trapping, flood damage reduction, an areas on a 100 percent user-pay basis. No formal water quality improvement/ riparian mapping effort has been initiated. The NWI is control, and ground water recharge. congressionally mandated to identify, classify, and digi- Wetlands have long been recognized as tize all wetlands and deepwater habitats in the United highly productive habitats for threat- States. For purposes of riparian mapping, the NWI has ened and endangered fish and wildlife developed a riparian definition that incorporates biologi- species. Wetlands provide habitat for cal information consistent with many agencies and 60 to 70 percent of the animal species applies information according to cartographic principles. federally listed as threatened or endan- For NWI mapping and classification purposes, a final def- gered (Lohoefner 1997). inition for riparian has been developed: The Federal Geographic Data Commit- Riparian areas are plant communities contiguous to and tee has adopted the U.S. Fish and affected by surface and subsurface hydrological features Wildlife Service’s Classification of Wet- of perennial or intermittent lotic and lentic water bodies lands and Deepwater Habitats of the (rivers, streams, lakes, and drainage ways). Riparian areas United States (Cowardin et al. 1979) have one or both of the following characteristics: (1) dis- as the national standard for wetlands tinctly different vegetative species than adjacent areas; classification. The Service’s National and (2) species similar to adjacent areas but exhibiting Wetlands Inventory (NWI) uses this more vigorous or robust growth forms. Riparian areas system to carry out its congressionally are usually transitional between wetland and upland. mandated role of identifying, classify- The definition applies primarily to regions of the lower ing, mapping, and digitizing data on 48 states in the arid west where the mean annual pre- wetlands and deepwater habitats. This cipitation is 16 inches or less and the mean annual evap- system, which defines wetlands consis- oration exceeds mean annual precipitation. For purposes tently with the National Academy of of this mapping, the riparian system is subdivided into Science’s reference definition, includes subsystems, classes, subclasses, and dominance types. Marine, Estuarine, Riverine, Lacustrine, (USFWS 1997) and Palustrine systems. The NWI has also developed protocols for classifying and mapping riparian habitats in the 22 coterminous western states. tats with water containing ocean- derived salts in excess of 0.5 parts per The riverine system under Cowardin’s thousand (ppt). classification includes all wetlands and deepwater habitats contained within a It is bounded on the upstream end by channel except wetlands dominated by uplands and on the downstream end at trees, shrubs, persistent emergents, the interface with tidal wetlands having emergent mosses, or lichens and habi- a concentration of ocean-derived salts that exceeds 0.5 ppt. Riverine wetlands

Biological Community Characteristics 2–61 are bounded perpendicularly on the standards against which a wetland is landward side by upland, the channel evaluated (Brinson 1995). bank (including natural and manufac- Under the HGM approach, riverine wet- tured levees), or by Palustrine wetlands. lands occur in floodplains and riparian In braided streams, riverine wetlands corridors associated with stream chan- are bounded by the banks forming the nels. The dominant water sources are outer limits of the depression within overbank flow or subsurface connec- which the braiding occurs. tions between stream channel and wet- Vegetated floodplain wetlands of the lands. Riverine wetlands lose water by river corridor are classified as Palustrine surface and subsurface flow returning to under this system. The Palustrine sys- the stream channel, ground water tem was developed to group the vege- recharge, and evapotranspiration. At the tated wetlands traditionally called by extension closest to the headwaters, such names as marsh, swamp, bog, fen, riverine wetlands often are replaced by and prairie pothole and also includes slope or depressional wetlands where small, shallow, permanent, or intermit- channel bed and bank disappear, or tent water bodies often called ponds. they may intergrade with poorly drained Palustrine wetlands may be situated flats and uplands. Usually forested, they shoreward of lakes, river channels, or extend downstream to the intergrade , on river floodplains, in iso- with estuarine fringe wetlands. Lateral lated catchments, or on slopes. They extent is from the edge of the channel also may occur as islands in lakes or perpendicularly to the edge of the flood- rivers. The Palustrine system includes all plain. In some landscape situations, nontidal wetlands dominated by trees, riverine wetlands may function hydro- shrubs, persistent emergents, emergent logically more like slope wetlands, and mosses and lichens, and all such wet- in headwater streams with little or no lands that occur in tidal areas where floodplain, slope wetlands may lie adja- salinity due to ocean-derived salts is cent to the stream channel (Brinson et below 0.5 ppt. The Palustrine system is al. 1995). Table 2.11 summarizes func- bounded by upland or by any of the tions of riverine wetlands under the other four systems. They may merge HGM approach. The U.S. Fish and with non-wetland riparian habitat Wildlife Service is testing an operational where hydrologic conditions cease to draft set of hydrogeomorphic type de- support wetland vegetation or may be scriptors to help bridge the gap between totally absent where hydrologic condi- the Cowardin system and the HGM ap- tions do not support wetlands at all proach (Tiner 1997). (Cowardin et al. 1979). For purposes of regulation under Sec- The hydrogeomorphic (HGM) approach is tion 404 of the Clean Water Act, only a system that classifies wetlands into areas with wetland hydrology, hy- similar groups for conducting functional drophytic vegetation, and hydric soils assessments of wetlands. Wetlands are are classified as regulated wetlands. classified based on geomorphology, As such, they represent a subset of the water source, and hydrodynamics. This areas classified as wetlands under the allows the focus to be placed on a Cowardin system. However, many areas group of wetlands that function much classified as wetlands under the Cow- more similarly than would be the case ardin system, but not classified as wet- without classifying them. Reference wet- lands for purposes of Section 404, are lands are used to develop reference nevertheless subject to regulation be-

2–62 Chapter 2: Stream Corridor Processes, Characteristics, and Functions cause they are part of the Waters of the Hydrologic Dynamic surface water storage United States. Long-term surface water storage Subsurface storage of water Aquatic Vegetation and Fauna Energy dissipation Stream biota are often classified in seven Moderation of ground-water flow or discharge groups—bacteria, algae, macrophytes Biogeochemical Nutrient cycling (higher plants), protists (amoebas, fla- Removal of elements and compounds gellates, ciliates), microinvertebrates Retention of particulates (invertebrates less than 0.02 inch in Organic carbon export length, such as rotifers, copepods, ostra- Plant habitat Maintain characteristic plant communities cods, and nematodes), macroinverte- Maintain characteristic detrital biomass brates (invertebrates greater than 0.02 Animal habitat Maintain spatial habitat structure inch in length, such as mayflies, stone- flies, caddisflies, crayfish, worms, Maintain interspersion and connectivity clams, and snails), and vertebrates Maintain distribution and abundance of invertebrates (fish, amphibians, reptiles, and mam- Maintain distribution and abundance of vertebrates mals) (Figure 2.33). The discussion of the River Continuum Concept in Table 2.11: Functions of riverine wetlands. Source: Brinson et al. 1995. Chapter 1, provides an overview of the major groups of organisms found in micro- and macroinvertebrates (Ruttner streams and how these assemblages 1963). Planktonic plant forms are usu- change from higher order to lower ally limited but may be present where order streams. the watershed contains lakes, ponds, Undisturbed streams can contain a re- floodplain waters, or slow current areas markable number of species. For exam- (Odum 1971). ple, a comprehensive inventory of The benthic invertebrate community of stream biota in a small German stream, streams may contain a variety of biota, the Breitenbach, found more than 1,300 including bacteria, protists, rotifers, bry- species in a 1.2-mile reach. Lists of ozoans, worms, crustaceans, aquatic in- algae, macroinvertebrates, and fish likely sect larvae, mussels, clams, crayfish, and to be found at potential restoration sites other forms of invertebrates. Aquatic in- may be obtained from state or regional vertebrates are found in or on a multi- inventories. The densities of such stream tude of microhabitats in streams biota are shown in Table 2.12. including plants, woody debris, rocks, Aquatic plants usually consist of algae interstitial spaces of hard substrates, and and mosses attached to permanent soft substrates (gravel, sand, and muck). stream substrates. Rooted aquatic vege- Invertebrate habitats exist at all vertical tation may occur where substrates are strata including the water surface, the suitable and high currents do not scour water column, the bottom surface, and the stream bottom. Luxuriant beds of deep within the hyporheic zone. vascular plants may grow in some areas Unicellular organisms and microinver- such as spring-fed streams in Florida tebrates are the most numerous biota in where water clarity, substrates, nutrients, streams. However, larger macroinverte- and slow water velocities exist. Bedrock brates are important to community or stones that cannot be moved easily structure because they contribute signif- by stream currents are often covered by icantly to a stream’s total invertebrate mosses and algae and various forms of biomass (Morin and Nadon 1991,

Biological Community Characteristics 2–63 light course particulate organic larger plants matter (mosses, red algae)

microorganisms epilithic (e.g., hyphomycete algae fungi)

dissolved organic matter microorganisms flocculation

fine particulate organic invertebrate matter invertebrate shredders scrapers

invertebrate collectors vertebrate Figure 2.33: Stream invertebrate predators predators biota. Food relation- ships typically found n streams.

Bourassa and Morin 1995). Further- effect on the abundance and taxonomic more, the larger species often play im- composition of algae and periphyton in portant roles in determining community streams. Likewise, macroinvertebrate composition of other components of predators, such as stoneflies, can influ- the ecosystem. For example, herbivo- ence the abundance of other species rous feeding activities of caddisfly lar- within the invertebrate community vae (Lamberti and Resh 1983), snails (Peckarsky 1985). (Steinman et al. 1987), and crayfish Collectively, microorganisms (fungi (Lodge 1991) can have a significant and bacteria) and benthic invertebrates

Table 2.12: Ranges of densities commonly facilitate the breakdown of organic ma- observed for selected groups of stream biota. terial, such as leaf litter, that enters the stream from external sources. Some Biotic Density invertebrates (insect larvae and am- Component (Individuals/Square Mile) phipods) act as shredders whose feed- Algae 109 – 1010 ing activities break down larger organic Bacteria 1012 – 1013 leaf litter to smaller particles. Other in- Protists 108 – 109 vertebrates filter smaller organic mater- Microinvertebrates 103 – 105 ial from the water (blackfly larvae, Macroinvertebrates 104 – 105 some mayfly nymphs, and some caddis- Vertebrates 100 – 102 fly larvae), scrape material off surfaces

2–64 Chapter 2: Stream Corridor Processes, Characteristics, and Functions (snails, limpets, and some caddisfly and tinction during and following the Pleis- mayfly nymphs), or feed on material tocene Age (Fausch et al. 1984). For ex- deposited on the substrate (dipteran ample, 210 species are found west of the larvae and some mayfly nymphs) (Moss Continental Divide, but only 40 of 1988). These feeding activities result in these species are found on both sides of the breakdown of organic matter in ad- the continent (Minckley and Douglas dition to the elaboration of invertebrate 1991). The relatively depauperate fauna tissue, which other consumer groups, of the Western United States has been such as fish, feed on. attributed to the isolating mechanisms Benthic macroinvertebrates, particularly of tectonic geology. Secondary biologi- aquatic insect larvae and crustaceans, cal, physical, and chemical factors may are widely used as indicators of stream further reduce the species richness of a health and condition. Many fish species specific community (Minckley and rely on benthic organisms as a food Douglas 1991, Allan 1995). source either by direct browsing on the Fish species assemblages in streams will benthos or by catching benthic organ- vary considerably from the headwaters isms that become dislodged and drift to the outlet due to changes in many downstream (Walburg 1971). hydrologic and geomorphic factors Fish are ecologically important in which control temperature, dissolved stream ecosystems because they are usu- oxygen, gradient, current velocity, and ally the largest vertebrates and often are substrate. Such factors combine to de- the apex predator in aquatic systems. termine the degree of habitat diversity The numbers and species composition in a given stream segment. Fish species of fishes in a given stream depends on richness tends to increase downstream the geographic location, evolutionary as gradient decreases and stream size history, and such intrinsic factors as increases. Species richness is generally physical habitat (current, depth, sub- lowest at small headwater streams due strates, riffle/pool ratio, wood snags, to increased gradient and small stream and undercut banks), water quality size, which increases the frequency and (temperature, dissolved oxygen, sus- severity of environmental fluctuations pended solids, nutrients, and toxic (Hynes 1970, Matthews and Styron chemicals), and biotic interactions (ex- 1980). In addition, the high gradient ploitation, predation, and competition). and decreased links with tributaries re- duces the potential for colonization There are approximately 700 native and entry of new species. freshwater species of fish in North America (Briggs 1986). Fish species Species richness increases in mid-order richness is highest in the Mississippi to lower stream reaches due to in- River Basin where most of the adaptive creased environmental stability, greater radiations have occurred in the United numbers of potential habitats, and in- States (Allan 1995). In the Midwest, as creases in numbers of colonization many as 50 to 100 species can occur in sources or links between major a local area, although typically only half drainages. As one proceeds down- the species native to a region may be stream, pools and runs increase over rif- found at any one location (Horwitz fles, allowing for an increase in fine 1978). Fish species richness generally bottom materials and facilitating the declines as one moves westward across growth of macrophytic vegetation. the United States, primarily due to ex- These environments allow for the pres- ence of fishes more tolerant of low oxy-

Biological Community Characteristics 2–65 gen and increased temperatures. Fur- Species generally may be referred to as ther, the range of body forms increases cold water or warm water, and grada- with the appearance of those species tions between, depending on their tem- with less fusiform body shapes, which perature requirements (Magnuson et al. are ecologically adapted to areas typi- 1979). Fish such as salmonids are usu- fied by decreased water velocities. In ally restricted to higher elevations or higher order streams or large rivers the northern climes typified by colder, bottom substrates often are typified by highly oxygenated water. These species finer sediments; thus herbivores, omni- tend to be specialists, with rather nar- vores, and planktivores may increase in row thermal tolerances and rather spe- response to the availability of aquatic cific reproductive requirements. For vegetation and plankton (Bond 1979). example, salmonids typically spawn by Fish have evolved unique feeding and depositing eggs over or within clean reproductive strategies to survive in the gravels which remain oxygenated and diverse habitat conditions of North silt-free due to upwelling of currents America. Horwitz (1978) examined the within the interstitial spaces. Reproduc- structure of fish feeding guilds in 15 tive movement and behavior is con- U.S. river systems and found that most trolled by subtle thermal changes fish species (33 percent) were benthic combined with increasing or decreasing insectivores, whereas piscivores (16 per- day-length. Salmonid populations, cent), herbivores (7 percent), omni- therefore, are highly susceptible to vores (6 percent), planktivores (3 many forms of habitat degradation, in- percent), and other guilds contained cluding alteration of flows, temperature, fewer species. However, Allan (1995) and substrate quality. indicated that fish frequently change Numerous fish species in the U.S. are feeding habits across habitats, life declining in number. Williams and stages, and season to adapt to changing Julien (1989) presented a list of North physical and biological conditions. Fish American fish species that the American in smaller headwater streams tend to be Fisheries Society believed should be insectivores or specialists, whereas the classified as endangered, threatened, or number of generalists and the range of of special concern. This list contains feeding strategies increases downstream 364 fish species warranting protection in response to increasing diversity of because of their rarity. Habitat loss was conditions. the primary cause of depletion for ap- Some fish species are migratory, return- proximately 90 percent of the species ing to a particular site over long dis- listed. This study noted that 77 percent tances to spawn. Others may exhibit of the fish species listed were found in great endurance, migrating upstream 25 percent of the states, with the high- against currents and over obstacles such est concentrations in eight southwestern as waterfalls. Many must move between states. Nehlsen et al. (1991) provided a salt water and freshwater, requiring list of 214 native naturally spawning great osmoregulatory ability (McKeown stocks of depleted Pacific salmon, steel- 1984). Species that return from the head, and sea-run cutthroat stocks from ocean environment into freshwater California, Oregon, Idaho, and Wash- streams to spawn are called anadromous ington. Reasons cited for the declines species. were alteration of fish passage and mi- gration due to dams, flow reduction as- sociated with hydropower and

2–66 Chapter 2: Stream Corridor Processes, Characteristics, and Functions agriculture, sedimentation and habitat that form and maintain them, are key loss due to logging and agriculture, to developing successful stream restora- overfishing, and negative interactions tion initiatives. with other fish, including nonnative The emphasis on fish community hatchery salmon and steelhead. restoration is increasing due to many The widespread decline in the numbers ecological, economic, and recreational of native fish species has led to current factors. In 1996 approximately 35 mil- widespread interest in restoring the lion Americans older than 16 partici- quality and quantity of habitats for fish. pated in recreational fishing, resulting Restoration activities have frequently in over $36 billion in expenditures centered on improving local habitats, (Brouha 1997). Much of this activity is such as fencing or removing livestock in streams, which justifies stream corri- from streams, constructing fish pas- dor restoration initiatives. sages, or installing instream physical While fish stocks often receive the great- habitat. However, research has demon- est public attention, preservation of strated that in most of these cases the other aquatic biota may also may be a success has been limited or question- goal of stream restoration. Freshwater able because the focus was too narrow mussels, many species of which are and did not address restoration of the threatened and endangered, are often of diverse array of habitat requirements particular concern. Mussels are highly and resources that are needed over the sensitive to habitat disturbances and life span of a species. obviously benefit from intact, well- Stream corridor restoration practition- managed stream corridors. The south- ers and others are now acutely aware central United States has the highest that fish require many different habitats diversity of mussels in the world. Mus- over the season and lifespan to fulfill sel ecology also is intimately linked needs for feeding, resting, avoiding with fish ecology, as fish function as predators, and reproducing. For exam- hosts for mussel larvae (glochidia). ple, Livingstone and Rabeni (1991) de- Among the major threats they face are termined that juvenile smallmouth bass dams, which lead to direct habitat loss in the Jacks Fork River of southeastern and fragmentation of remaining habi- Missouri fed primarily on small tat, persistent sedimentation, pesticides, macroinvertebrates in littoral vegeta- and introduced exotic species, such as tion. Vegetation represented not only a fish and other mussel species. source of food but a refuge from preda- tors and a warmer habitat, factors that Abiotic and Biotic Interrelations can collectively optimize chances for in the Aquatic System survival and growth (Rabeni and Jacob- Much of the spatial and temporal vari- son 1993). Adult smallmouth bass, ability of stream biota reflects variations however, tended to occupy deeper pool in both abiotic and biotic factors, in- habitats, and the numbers and biomass cluding water quality, temperature, of adults at various sites were attributed streamflow and flow velocity, substrate, to these specific deep-water habitats the availability of food and nutrients, (McClendon and Rabeni 1987). Rabeni and predator-prey relationships. These and Jacobson (1993) suggested that an factors influence the growth, survival, understanding of these specific habitats, and reproduction of aquatic organisms. combined with an understanding of the While these factors are addressed indi- fluvial hydraulics and geomorphology

Biological Community Characteristics 2–67 vidually below, it is important to re- titude, latitude, origin of the water, and member that they are often interdepen- solar radiation (Ward 1985, Sweeney dent. 1993). Temperature governs many bio- chemical and physiological processes in Flow Condition cold-blooded aquatic organisms be- The flow of water from upstream to cause their body temperature is the downstream distinguishes streams from same as the surrounding water; thus, other ecosystems. The spatial and tem- water temperature has an important poral characteristics of streamflow, such role in determining growth, develop- as fast versus slow, deep versus shallow, ment, and behavioral patterns. Stream turbulent versus smooth, and flooding insects, for example, often grow and de- versus low flows, are described previ- velop more rapidly in warmer portions ously in this chapter. These flow charac- of a stream or during warmer seasons. teristics can affect both micro- and Where the thermal differences among macro-distribution patterns of numer- sites are significant (e.g., along latitudi- ous stream species (Bayley and Li 1992, nal or altitudinal gradients), it is possi- Reynolds 1992, Ward 1992). Many or- ble for some species to complete two or ganisms are sensitive to flow velocity more generations per year at warmer because it represents an important sites; these same species complete one mechanism for delivering food and nu- or fewer generations per year at cooler trients yet also may limit the ability of sites (Sweeney 1984, Ward 1992). organisms to remain in a stream seg- Growth rates for algae and fish appear ment. Some organisms also respond to to respond to temperature changes in a temporal variations in flow, which can similar fashion (Hynes 1970, Reynolds change the physical structure of the 1992). The relationships between tem- stream channel, as well as increase mor- perature and growth, development, and tality, modify available resources, and behavior can be strong enough to affect disrupt interactions among species geographic ranges of some species (Resh et al. 1988, Bayley and Li 1992). (Table 2.13). The flow velocity in streams determines Water temperature is one of the most whether planktonic forms can develop important factors determining the dis- and sustain themselves. The slower the tribution of fish in freshwater streams, currents in a stream, the more closely due both to direct impacts and influ- the composition and configuration of ence on dissolved oxygen concentra- biota at the shore and on the bottom tions, and is influenced by local approach those of standing water (Rut- conditions, such as shade, depth and tner 1963). High flows are cues for tim- current. Many fish species can tolerate ing migration and spawning of some only a limited temperature range. Such fishes. High flows also cleanse and sort fish as salmonids and sculpins domi- streambed materials and scour pools. nate in cold water streams, whereas Extreme low flows may limit young fish such species as largemouth bass, small- production because such flows often mouth bass, suckers, minnows, sun- occur during periods of recruitment and fishes and catfishes may be present in growth (Kohler and Hubert 1993). warmer streams (Walburg 1971). Water Temperature Effects of Cover Water temperature can vary markedly For the purposes of restoration, land within and among stream systems as a use practices that remove overhead function of ambient air temperature, al-

2–68 Chapter 2: Stream Corridor Processes, Characteristics, and Functions Table 2.13: Maximum weekly average temperatures for growth and short term maximum temperatures for selected fish (ºF). Source: Brungs and Jones 1977.

Species Max. Weekly Max. Temp. for Max. Weekly Max. Temp. Average Temp. for Survival of Short Average Temp. for Embryo Growth (Juveniles) Exposure (Juveniles) for Spawninga Spawningb Atlantic salmon 68ºF73ºF 41ºF 52ºF Bluegill 90ºF95ºF77ºF93ºF Brook trout 66ºF75ºF48ºF55ºF Common carp 70ºF91ºF Channel catfish 90ºF95ºF81ºF84ºFc Largemouth bass 90ºF93ºF70ºF81ºFc Rainbow trout 66ºF75ºF48ºF55ºF Smallmouth bass 84ºF63ºF73ºFc Sockeye salmon 64ºF72ºF50ºF55ºF a Optimum or mean of the range of spawning temperatures reported for the species. b Upper temperature for successful incubation and hatching reported for the species. c Upper temperature for spawning. cover or decrease baseflows can increase order meadow stream than in a compa- instream temperatures to levels that ex- rable wooded reach from April through ceed critical thermal maxima for fishes October, the reverse was true from No- (Feminella and Matthews 1984). Thus, vember through March. In a review of maintenance or restoration of normal temperature effects on stream macroin- temperature regimes can be an impor- vertebrates common to the Pennsylva- tant endpoint for stream managers. nia Piedmont, Sweeney (1992) found Riparian vegetation is an important fac- that temperature changes of 2 to 6 ºC tor in the attenuation of light and tem- usually altered key life-history charac- perature in streams (Cole 1994). Direct teristics of the study species. Riparian sunlight can significantly warm streams, forest buffers have been shown to pre- particularly during summer periods of vent the disruption of natural tempera- low flow. Under such conditions, ture patterns as well as to mitigate the streams flowing through forests warm increases in temperature following de- rapidly as they enter deforested areas, forestation (Brown and Krygier 1970, but may also cool somewhat when Brazier and Brown 1973). streams reenter the forest. In Pennsylva- The exact buffer width needed for tem- nia (Lynch et al. 1980), average daily perature control will vary from site to stream temperatures that increased site depending on such factors as 12ºC through a clearcut area were sub- stream orientation, vegetation, and stantially moderated after flow through width. Along a smaller, narrow headwa- 1,640 feet of forest below the clearcut. ter stream, the reestablishment of They attributed the temperature reduc- shrubs, e.g., willows and alders, may tion primarily to inflows of cooler provide adequate shade and detritus to ground water. restore both the riparian and aquatic A lack of cover also affects stream tem- ecosystems. The planting and/or perature during the winter. Sweeney reestablishment of large trees, e.g., cot- (1993) found that, while average daily tonwoods, willows, sycamores, ash, and temperatures were higher in a second- walnuts (Lowe 1964), along larger, higher order rivers can improve the seg-

Biological Community Characteristics 2–69 ment of the fishery closest to the banks, less competitive in sustaining the but has little total effect on light and species (Mackenthun 1969). Dissolved temperature of wider rivers. oxygen concentrations of 3.0 mg/L or Heat budget models can accurately pre- less have been shown to interfere with dict stream and river temperatures (e.g., fish populations for a number of rea- Beschta 1984, Theurer et al. 1984). sons (Mackenthun 1969, citing several Solar radiation is the major factor influ- other sources) (Table 2.14). encing peak summer water tempera- Depletion of dissolved oxygen can re- tures and shading is critical to the sult in the death of aquatic organisms, overall temperature regime of streams including fish. Fish die when the de- in small watersheds. mand for oxygen by biological and chemical processes exceeds the oxygen Dissolved Oxygen input by reaeration and photosynthesis, Oxygen enters the water by absorption resulting in fish suffocation. Oxygen de- directly from the atmosphere and by pletion usually is associated with slow plant photosynthesis (Mackenthun current, high temperature, extensive 1969). Due to the shallow depth, large growth of rooted aquatic plants, algal surface exposure to air and constant blooms, or high concentrations of or- motion, streams generally contain an ganic matter (Needham 1969). abundant dissolved oxygen supply even when there is no oxygen production by Stream communities are susceptible to photosynthesis. pollution that reduces the dissolved oxygen supply (Odum 1971). Major Dissolved oxygen at appropriate con- factors determining the amount of oxy- centrations is essential not only to keep gen found in water are temperature, aquatic organisms alive but to sustain pressure, abundance of aquatic plants their reproduction, vigor, and develop- and the amount of natural aeration ment. Organisms undergo stress at re- from contact with the atmosphere duced oxygen levels that make them (Needham 1969). A level of 5 mg/L of

Table 2.14: Summary of dissolved oxygen concentrations (mg/L) generally associated with effects on fish in salmonid and nonsalmonid waters. Source: USEPA 1987.

Level of Effect Salmonida Nonsalmonid Early life stages (eggs and fry) No production impairment 11 (8) 6.5 Slight production impairment 9 (6) 5.5 Moderate production impairment 8 (5) 5.0 Severe production impairment 7 (4) 4.5 Limit to avoid acute mortality 6 (3) 4.0 Other life stages No production impairment 8 (0) 6.0 Slight production impairment 6 (0) 5.0 Moderate production impairment 5 (0) 4.0 Severe production impairment 4 (0) 3.5 Limit to avoid acute mortality 3 (0) 3.0

a Values for salmonid early life stages are water column concentrations recommended to achieve the required concentration of dissolved oxygen in the gravel spawning substrate (shown in parentheses).

2–70 Chapter 2: Stream Corridor Processes, Characteristics, and Functions dissolved oxygen in water is associated increased acidity of rainfall in some with normal activity of most fish (Wal- parts of the United States, especially burg 1971). Oxygen analyses of good areas downwind of industrial and trout streams show dissolved oxygen urban emissions (Schreiber 1995). Of concentrations that range from 4.5 to particular concern are environments 9.5 mg/L (Needham 1969). that have a reduced capacity to neutral- ize acid inputs because soils have a lim- pH ited buffering capacity. Acidic rainfall Aquatic organisms from a wide range of can be especially harmful to environ- taxa exist and thrive in aquatic systems ments such as the Adirondack region of with nearly neutral hydrogen ion activ- upstate New York, where runoff already ity (pH 7). Deviations, either toward a tends to be slightly acidic as a result of more basic or acidic environment, in- natural conditions. crease chronic stress levels and eventu- ally decrease species diversity and Substrate abundance (Figure 2.34). One of the Stream biota respond to the many abi- more widely recognized impacts of otic and biotic variables influenced by changes in pH has been attributed to substrate. For example, differences in

Figure 2.34: Effects of acid rain on some aquatic species. As acidity increases (and pH decreases) in lakes and streams, some species are lost.

Rainbow trout (Oncorhyncus mykiss) *embryonic life stage **selected species Brown trout (Salmo trutta) Brook trout (Salvelinus fontinalus) Smallmouth bass (Micropterus dolomieu) Flathead minnow (Pimephalus promelas) Pumpkinseed sunfish (Lepomis gibbosus) Yellow perch (Perca flavescens) Bullfrog* (Rana catesbeiana) Wood frog* (R. sylvatica) American toad* (Bufo americanus) Spotted salamander* (Ambystoma maculatum) Clam**

Crayfish**

Snail**

Mayfly**

6.56.0 5.5 5.0 4.5 4.0 3.5 3.0 pH

Biological Community Characteristics 2–71 species composition and abundance usually greatest, but it tends to be can be observed among macroinverte- highly discontinuous because of fea- brate assemblages found in snags, sand, tures associated with fluvial activities bedrock, and cobble within a single such as oxbow lakes and cutoff chan- stream reach (Benke et al. 1984, Smock nels, and because of complex interac- et al. 1985, Huryn and Wallace 1987). tions of local, intermediate, and This preference for conditions associ- regional ground water systems (Naiman ated with different substrates con- et al. 1994) (Figure 2.35). tributes to patterns observed at larger Stream substrates are composed of vari- spatial scales where different macroin- ous materials, including clay, sand, vertebrate assemblages are found in gravel, cobbles, boulders, organic mat- coastal, piedmont, and mountain ter, and woody debris. Substrates form streams (Hackney et al. 1992). solid structures that modify surface and Stream substrates can be viewed in the interstitial flow patterns, influence the same functional capacity as soils in the accumulation of organic materials, and terrestrial system; that is, stream sub- provide for production, decomposition, strates constitute the interface between and other processes (Minshall 1984). water and the hyporheic subsurface of Sand and silt are generally the least the aquatic system. The hyporheic zone favorable substrates for supporting is the area of substrate which lies below aquatic organisms and support the the substrate/water interface, and may fewest species and individuals. Flat or range from a layer extending only rubble substrates have the highest den- inches beneath and laterally from the sities and the most organisms (Odum stream channel, to a very large subsur- 1971). As previously described, sub- face environment. Alluvial floodplains strate size, heterogeneity, stability with of the Flathead River, Montana, have a respect to high and baseflow, and dura- hyporheic zone with significant sur- bility vary within streams, depending face water/ground water interaction on particle size, density, and kinetic en- which is 2 miles wide and 33 feet deep ergy of flow. Inorganic substrates tend (Stanford and Ward 1988). Naiman et to be of larger size upstream than downstream al. (1994) discussed the extent and con- and tend to be larger in riffles than in nectivity of hyporheic zones around pools (Leopold et al. 1964). Likewise, streams in the Pacific Northwest. They the distribution and role of woody de- hypothesized that as one moves from bris varies with stream size (Maser and low-order (small) streams to high-order Sedell 1994). (large) streams, the degree of hy- In forested watersheds, and in streams porheic importance and continuity with significant areas of trees in their ri- first increases and then decreases. In parian corridor, large woody debris that small streams, the hyporheic zone is falls into the stream can increase the limited to small floodplains, meadows, quantity and diversity of substrate and and stream segments where coarse sedi- aquatic habitat or range (Bisson et al. ments are deposited over bedrock. The 1987, Dolloff et al. 1994). Debris dams hyporheic zones are generally not con- trap sediment behind them and often tinuous. In mid-order channels with create scour holes immediately down- more extensive floodplains, the spatial stream. Eroded banks commonly occur connectivity of the hyporheic zone in- at the boundaries of debris blockages. creases. In large order streams, the spa- tial extent of the hyporheic zone is

2–72 Chapter 2: Stream Corridor Processes, Characteristics, and Functions Organic Material Metabolic activity within a stream reach water table depends on autochthonous, allochtho- nous, and upstream sources of food and nutrients (Minshall et al. 1985). Au- tochthonous materials, such as algae and aquatic macrophytes, originate within the stream channel, whereas al- lochthonous materials such as wood, permeable leaves, and dissolved organic carbon, layer originate outside the stream channel. Upstream materials may be of au- hyporheic tochthonous or allochthonous origin zone and are transported by streamflow to ground water downstream locations. Seasonal flood- ing provides allochthonous input of or- impermeable layer ganic material to the stream channel and also can significantly increase the rate of decomposition of organic material. Figure 2.35: Hyporheic zone. Summary of the The role of primary productivity of different means of migration undergone by streams can vary depending on geo- members of the stream benthic community. graphic location, stream size, and sea- son (Odum 1957, Minshall 1978). The gen reserves and result in fish kills and river continuum concept (Vannote et al. other aesthetic problems in waterbodies. 1980) (see The River Continuum Concept Eutrophication in lakes and reservoirs is in section 1.E in Chapter 1) hypothe- indirectly measured as standing crops sizes that primary productivity is of of phytoplankton biomass, usually rep- minimal importance in shaded head- resented by planktonic chlorophyll a water streams but increases in signifi- concentration. However, phytoplankton cance as stream size increases and biomass is usually not the dominant riparian vegetation no longer limits the portion of plant biomass in smaller entry of light to stream periphyton. Nu- streams, due to periods of energetic merous researchers have demonstrated flow and high substrate to volume ra- that primary productivity is of greater tios that favor the development of peri- importance in certain ecosystems, in- phyton and macrophytes on the stream cluding streams in grassland and desert bottom. Stream eutrophication can re- ecosystems. Flora of streams can range sult in excessive algal mats and oxygen from diatoms in high mountain streams depletion at times of decreased flows to dense stands of macrophytes in low and higher temperatures (Figure 2.36). gradient streams of the Southeast. Furthermore, excessive plant growth can As discussed in Section 2.C, loading of occur in streams at apparently low am- nitrogen and phosphorus to a stream bient concentrations of nitrogen and can increase the rate of algae and phosphorus because the stream currents aquatic plant growth, a process known promote efficient exchange of nutrients as eutrophication. Decomposition of this and metabolic wastes at the plant cell excess organic matter can deplete oxy- surface.

Biological Community Characteristics 2–73 stream portions of the stream. This uni- directional movement of nutrients and organic matter in lotic systems is slowed by the temporary retention, storage, and utilization of nutrients in leaf packs, accumulated debris, inverte- brates, and algae. Organic matter processing has been shown to have nutrient-dependent rela- tionships similar to primary productiv- ity. Decomposition of leaves and other forms of organic matter can be limited by either nitrogen or phosphorus, with predictive N:P ratios being similar to those for growth of algae and periphy- ton. Leaf decomposition occurs by a sequential combination of microbial Figure 2.36: Stream eutrophication. decomposition, invertebrate shredding, Eutrophication can result in oxygen depletion. and physical fractionation. Leaves and organic matter itself are generally low In many streams, shading or turbidity in protein value. However, the coloniza- limit the light available for algal tion of organic matter by bacteria and growth, and biota depend highly on fungi increases the net content of nitro- allochthonous organic matter, such as gen and phosphorus due to the accu- leaves and twigs produced in the sur- mulation of proteins and lipids rounding watershed. Once leaves or contained in microbial biomass. These other allochthonous materials enter the compounds are a major nutritive source stream, they undergo rapid changes for aquatic invertebrates. Decaying or- (Cummins 1974). Soluble organic com- ganic matter represents a major storage pounds, such as sugars, are removed via component for nutrients in streams, as leaching. Bacteria and fungi subse- well as a primary pathway of energy quently colonize the leaf materials and and nutrient transfer within the food metabolize them as a source of carbon. web. Ultimately, the efficiency of reten- The presence of the microbial biomass tion and utilization is reflected at the increases the protein content of the top of the food web in the form of fish leaves, which ultimately represents a biomass. high quality food resource for shred- Organisms often respond to variations ding invertebrates. in the availability of autochthonous, al- The combination of microbial decom- lochthonous, and upstream sources. For position and invertebrate shredding/ example, herbivores are relatively more scraping reduces the average particle common in streams having open ripar- size of the organic matter, resulting in ian canopies and high algal productiv- ity compared to streams having closed the loss of carbon both as respired CO2 and as smaller organic particles trans- canopies and accumulated leaves as the ported downstream. These finer parti- primary food resource (Minshall et al. cles, lost from one stream segment, 1983). Similar patterns can be observed become the energy inputs to the down- longitudinally within the same stream (Behmer and Hawkins 1986).

2–74 Chapter 2: Stream Corridor Processes, Characteristics, and Functions Terrestrial and Aquatic aquatic systems. As the character and Ecosystem Components for distribution of vegetation is altered by Stream Corridor Restoration removal of biomass, agriculture, live- stock grazing, development, and other The previous sections presented the bio- land uses, and the flow patterns of logical components and functional water, sediment, and nutrients are mod- processes that shape stream corridors. ified, the interactions among system The terrestrial and aquatic environ- components become less efficient and ments were discussed separately for the effective. These problems can become sake of simplicity and ease of under- more pronounced when they are aggra- standing. Unfortunately, this is fre- vated by introductions of excess nutri- quently the same approach taken in ents and synthetic toxins, soil environmental restoration initiatives, disturbances, and similar impacts. with efforts placed separately on the uplands, riparian area, or instream Stream migration and flooding are channel. The stream corridor must be principal sources of structural and viewed as a single functioning unit or compositional variation within and ecosystem with numerous connections among plant communities in most and interactions between components. undisturbed floodplains (Brinson et al. Successful stream corridor restoration 1981). Although streams exert a com- cannot ignore these fundamental rela- plex influence on plant communities, tionships. vegetation directly affects the integrity and characteristics of stream systems. The structure and functions of vegeta- For example, root systems bind bank tion are interrelated at all scales. They sediments and moderate erosion are also directly tied to ecosystem dy- processes, and floodplain vegetation namics. Particular vegetation types may slows overbank flows, inducing sedi- have characteristic regeneration strate- ment deposition. Trees and smaller gies (e.g., fire, treefall gaps) that main- woody debris that fall into the channel tain those types within the landscape at deflect flows, inducing erosion at some all times. Similarly, certain topographic points and deposition at others, alter settings may be more likely than others pool distribution, the transport of or- to be subject to periodic, dramatic ganic material, as well as a number of changes in hydrology and related vege- other processes. The stabilization of tation structure as a result of massive streams that are highly interactive with debris jams or occupation by beavers. their floodplains can disrupt the funda- However, in the context of stream corri- mental processes controlling the struc- dor ecosystems, some of the most fun- ture and function of stream corridor damental dynamic interactions relate to ecosystems, thereby indirectly affecting stream flooding and channel migration. the characteristics of the surrounding Many ecosystem functions are influ- landscape. enced by the structural characteristics of In most instances, the functions of veg- vegetation. In an undeveloped water- etation that are most apparent are those shed, the movement of water and other that influence fish and wildlife. At the materials is moderated by vegetation landscape level, the fragmentation of and detritus, and nutrients are mobi- native cover types has been shown to lized and conserved in complex pat- significantly influence wildlife, often fa- terns that generally result in balanced voring opportunistic species over those interactions between terrestrial and requiring large blocks of contiguous

Biological Community Characteristics 2–75 habitat. In some systems, relatively and Johnson 1971, Johnson 1971, small breaks in corridor continuity can Carothers et al. 1974). Subsequent have significant impacts on animal studies by other investigators found movement or on the suitability of similar results. Basically, cottonwood- stream conditions to support certain willow gallery forests of the North aquatic species. In others, establishment American Southwest supported the of corridors that are structurally differ- highest concentrations of noncolonial ent from native systems or inappropri- nesting birds for North America. De- ately configured can be equally struction and fragmentation of these ri- disruptive. Narrow corridors that are es- parian forests reduced species richness sentially edge habitat may encourage and resulted in a nearly straight-line re- generalist species, nest parasites, and lationship between numbers of nesting predators, and where corridors have pairs/acre and number of mature been established across historic barriers trees/acre. Later studies demonstrated to animal movement, they can disrupt that riparian areas are equally impor- the integrity of regional animal assem- tant as conduits for migrating birds blages (Knopf et al. 1988). (Johnson and Simpson 1971, Stevens et Some riparian dependent species are al. 1977). linked to streamside riparian areas with When considering restoration of ripar- fairly contiguous dense tree canopies. ian habitats, the condition of adjacent Without new trees coming into the habitats must be considered. Carothers population, older trees creating this (1979) found that riparian ecosystems, linked canopy eventually drop out, cre- especially the edges, are widely used by ating ever smaller patches of habitat. nonriparian birds. In addition he found Restoration that influences tree stands that some riparian birds utilized adja- so that sufficient recruitment and patch cent nonriparian ecosystems. Carothers size can be attained will benefit these et al. (1974) found that smaller breed- species. For similar reasons, many ripar- ing species [e.g., warblers and the West- ian-related raptors such as the common ern wood pewee (Contopus sordidulus)] black-hawk (Buteogallus anthracinus), tended to carry on all activities within gray hawk (Buteo nitidus), bald eagle the riparian ecosystem during the (Haliaeetus leucocephalus), Cactus ferrug- breeding season. However, larger inous pygmy-owl (Glaucidium brasil- species (e.g., kingbirds and doves) com- ianum cactorum), and Cooper’s hawk monly foraged outside the riparian (Accipiter cooperii), depend upon various ecosystem in adjacent habitats. Larger sizes and shapes of woody riparian trees species (e.g., raptors) may forage miles for nesting substrate and roosts. from riparian ecosystems, but still de- Restoration practices that attain suffi- pend on them in critical ways (Lee et al. cient tree recruitment will greatly bene- 1989). fit these species in the long term, and Because of more mesic conditions cre- other species in the short term. ated by the canyon effect, canyons and Some aspects related to this subject their attendant riparian vegetation serve have been discussed as ecosystem com- as corridors for short-range movements ponents and functions under other sec- of animals along elevational gradients tions. Findings from the earliest studies (e.g., between summer and winter of the impacts of fragmentation of ri- ranges). Long-range movements that parian habitats on breeding birds were occur along riparian zones throughout published for the Southwest (Carothers North America include migration of

2–76 Chapter 2: Stream Corridor Processes, Characteristics, and Functions birds and bats. Riparian zones also braska has greatly decreased the serve as stopover habitat for migrating amount of important wet meadow birds (Stevens et al. 1977). Woody vege- habitat. This area has been declared tation is generally important, not only critical habitat for the whooping crane to most riparian ecosystems, but also to (Grus americana) (Aronson and Ellis adjacent aquatic and even upland 1979), for piping plover, and for the in- ecosystems. However, it is important to terior least tern. It is also an important establish clear management objectives staging area for up to 500,000 sandhill before attempting habitat modification. cranes (Grus canadensis) from late Feb- Restoring all of a given ecosystem to its ruary to late April and supports 150 to “pristine condition” may be impossible, 250 bald eagles (Haliaeetus especially if upstream conditions have leucocephalus). Numerous other impor- been heavily modified, such as by a tant species using the area include the dam or other water diversion project. peregrine falcon (Falco peregrinus), Even if complete restoration is a possi- Canada goose (Branta canadensis), mal- bility, it may not accomplish or com- lard (Anas platyrhynchos), numerous plement the restoration goals. other waterfowl, and raptors (USFWS 1981). Thus, managers here are con- For example, encroachment of woody fronted with means of reducing riparian vegetation in the channel below several groves in favor of wet meadows. dams in the Platte River Valley in Ne-

Biological Community Characteristics 2–77 2.E Functions and Dynamic Equilibrium

Throughout the past two chapters, this Habitat—the spatial document has covered stream corridor structure of the envi- structure and the physical, chemical, ronment which allows species to live, repro- and biological processes occurring in duce, feed, and move. stream corridors. This information shows how stream corridors function as Habitat ecosystems, and consequently, how these characteristic structural features Barrier—the stoppage and processes must be understood in of materials, energy, order to enable stream corridor func- and organisms. tions to be effectively restored. In fact, reestablishing structure or restoring a particular physical or biological process is not the only thing that restoration Barrier seeks to achieve. Restoration aims to Conduit—the ability of reestablish valued functions. Focusing the system to transport on ecological functions gives the materials, energy, and restoration effort its best chance to organisms. recreate a self-sustaining system. This property of sustainability is what sepa- rates a functionally sound stream, that Conduit freely provides its many benefits to peo- Filter—the selective ple and the natural environment, from penetration of materi- an impaired watercourse that cannot als, energy, and organ- sustain its valued functions and may re- isms. main a costly, long-term maintenance burden. Filter Section 1.A of Chapter 1 emphasized matrix, patch, corridor and mosaic as Source—a setting the most basic building blocks of physi- where the output of cal structure at local to regional scales. materials, energy, and Ecological functions, too, can be sum- organisms exceeds marized as a set of basic, common input. themes that recur in an infinite variety of settings. These six critical functions Source are habitat, conduit, filter, barrier, source, Sink—a setting where and sink (Figure 2.37). the input of water, In this section, the processes and struc- energy, organisms and materials exceeds tural descriptions of the past two chap- output. ters are revisited in terms of these critical ecological functions. Sink

Two attributes are particularly impor- Figure 2.37: Critical ecosystem functions. Six tant to the operation of stream corridor functions can be summarized as a set of basic, functions: common themes recurring in a variety of settings.

2–78 Chapter 2: Stream Corridor Processes, Characteristics, and Functions Connectivity—This is a measure of average dimension and variance, how spatially continuous a corridor number of narrows, and varying or a matrix is (Forman and Godron habitat requirements (Dramstad et 1986). This attribute is affected by al. 1996). gaps or breaks in the corridor and Width and connectivity interact between the corridor and adjacent throughout the length of a stream corri- land uses (Figure 2.38). A stream dor. Corridor width varies along the corridor with a high degree of con- length of the stream and may have nectivity among its natural commu- gaps. Gaps across the corridor interrupt nities promotes valuable functions and reduce connectivity. Evaluating including transport of materials and connectivity and width can provide energy and movement of flora and some of the most valuable insight for fauna. designing restoration actions that miti- Width—In stream corridors, this refers gate disturbances. to the distance across the stream and The following subsections discuss each its zone of adjacent vegetation cover. of the functions and general relation- Factors affecting width are edges, ship to connectivity and width. The community composition, environ- final subsection discusses dynamic mental gradients, and disturbance equilibrium and its relevance to stream effects of adjacent ecosystems, corridor restoration. including those with human activity. Example measures of width include

AB

Figure 2.38: Landscapes with (A) high and (B) low degrees of connectivity. A connected landscape structure generally has higher levels of functions than a fragmented landscape.

Functions and Dynamic Equilibrium 2–79 Habitat Functions major river valleys together can provide substantial habitat. North American fly- ways include examples of stream and river corridor habitat exploited by mi- gratory birds at landscape to regional scales. Stream corridors, and other types of naturally vegetated corridors as well, can provide migrating forest and ripar- Habitat is a term used to describe an ian species with their preferred resting area where plants or animals (including and feeding habitats during migration people) normally live, grow, feed, re- stopovers. Large mammals such as produce, and otherwise exist for any black bear are known to require large, portion of their life cycle. Habitats pro- contiguous wild terrain as home range, vide organisms or communities of or- and in many parts of the country broad ganisms with the necessary elements of stream corridors are crucial to linking life, such as space, food, water, and smaller patches into sufficiently large shelter. territories. Under suitable conditions often pro- Habitat functions within watersheds vided by stream corridors, many species may be examined from a somewhat dif- can use the corridor to live, find food ferent perspective. Habitat types and and water, reproduce, and establish vi- patterns within the watershed are signif- able populations. Some measures of a icant, as are patterns of connectivity to stable biological community are popu- adjoining watersheds. The vegetation of lation size, number of species, and ge- the stream corridor in upper reaches of netic variation, which fluctuate within watersheds sometimes has become dis- expected limits over time. To varying connected from that of adjacent water- degrees, stream corridors constructively sheds and corridors beyond the divide. influence these measures. The corridor’s When terrestrial or semiaquatic stream value as habitat is increased by the fact corridor communities are connected at that corridors often connect many small their headwaters, these connections will habitat patches and thereby create usually help provide suitable alternative larger, more complex habitats with habitats beyond the watershed. larger wildlife populations and higher Assessing habitat function at the stream biodiversity. corridor and smaller scales can also be Habitat functions differ at various viewed in terms of patches and corri- scales, and an appreciation of the scales dors, but in finer detail than in land- at which different habitat functions scapes and watersheds. It is also at local occur will help a restoration initiative scales that transitions among the vari- succeed. The evaluation of habitat at ous habitats within the corridor can be- larger scales, for example, may make come more important. Stream corridors note of a biotic community’s size, com- often include two general types of habi- position, connectivity, and shape. tat structure: interior and edge habitat. Habitat diversity is increased by a corri- At the landscape scale, the concepts of dor that includes both edge and interior matrix, patches, mosaics and corridors conditions, although for most streams, are often involved in describing habitat corridor width is insufficient to provide over large areas. Stream corridors and

2–80 Chapter 2: Stream Corridor Processes, Characteristics, and Functions edge is more pronounced when the amount of interior habitat is minimal. Two important habitat characteristics are edges and interior (Figure 2.39) Edges are critical lines of Interior habitat occurs further from the perimeter interaction between different ecosystems. Interior of the element. Interior is typified by more stable habitats are generally more stable, sheltered envi- environmental inputs than those found at the ronments where the ecosystem may remain rela- edge of an ecosystem. Sunlight, rainfall, and wind tively the same for prolonged periods. Edge habi- effects are less intense in the interior. Many sensi- tat is exposed to highly variable environmental gra- tive or rare species depend upon a less-disturbed dients. The result is a different species composition environment for their survival. They are therefore and abundance than observed interior habitat. tolerant of only “interior” habitat conditions. The Edges are important as filters of disturbance to distance from the perimeter required to create interior habitat. Edges can also be diverse areas these interior conditions is dependent upon the with a large variety of flora and fauna. species’ requirements. Edges and interiors are scale-independent concepts. Interior plants and animals differ considerably from Larger mammals known as interior forest species those that prefer or tolerate the edge’s variability. may need to be miles from the forest edge to find With an abundance of edge, stream corridors desired habitat, while an insect or amphibian may often have mostly edge species. Because large be sensitive to the edges and interiors of the micro- ecosystems and wide corridors are becoming habitat under a rotted log. The edges and interiors increasingly fragmented in modern landscapes, of a stream corridor, therefore, depend upon the however, interior species are often rare and hence species being considered. As elongated, narrow are targets for restoration. The habitat require- ecosystems that include land/water interfaces and ments of interior species (with respect to distance often include natural/human-made boundaries as from edge are a useful guide in restoring larger well at the upland fringe, stream corridors have an stream corridors to provide a diversity of habitat abundance of edges and these have a pronounced types and sustainable communities. effect on their biota. Edges and interiors are each preferred by different sets of plant and animal species, and it is inappro- priate to consider edges or interiors as consistently “bad” or “good” habitat characteristics. It may be desirable to maintain or increase edge in some circumstances, or favor interior habitats in others. Generally speaking, however, human activity tends to increase edge and decrease interior, so more often it is restoring or protecting interior that merits specific management action. Edge habitat at the stream corridor boundary typi- edge cally has higher inputs of solar energy, precipita- tion, wind energy, and other influences from the interior adjacent ecosystems. The difference in environ- mental gradients at the stream corridor’s edge Figure 2.39: Edge and interior habitat of a woodlot. results in a diversified plant and animal community Interior plants and animals differ considerably from interacting with adjacent ecosystems. The effect of those that prefer or tolerate the edge’s variability.

Natural Disturbances 2–81 much interior habitat for larger verte- Pools may be formed downstream from brates such as forest interior bird a log that has fallen across a stream and species. For this reason, increasing inte- both upstream and downstream flow rior habitat is sometimes a watershed characteristics are altered. The structure scale restoration objective. formed by large woody debris in a Habitat functions at the corridor scale stream improves aquatic habitat for are strongly influenced by connectivity most fish and invertebrate species. and width. Greater connectivity and in- Riparian forests, in addition to their creased width along and across a stream edge and interior habitats, may offer corridor generally increases its value as vertical habitat diversity in their canopy, habitat. Stream valley morphology and subcanopy, shrub and herb layers. And environmental gradients (such as grad- within the channel itself, riffles, pools, ual changes in soil wetness, solar radia- glides, rapids and backwaters all pro- tion, and precipitation) can cause vide different habitat conditions in changes in plant and animal communi- both the water column and the ties. More species generally find suitable streambed. These examples, all de- habitat conditions in a wide, contigu- scribed in terms of physical structure, ous, and diverse assortment of native illustrate once again the strong linkage plant communities within the stream between structure and habitat function. corridor than in a narrow, homoge- neous or highly fragmented corridor. Conduit Function When applied strictly to stream chan- nels, however, this might not be true. Some narrow and deeply incised streams, for example, provide thermal conditions that are critical for endan- gered salmonids. Habitat conditions within a corridor vary according to factors such as climate The conduit function is the ability to and microclimate, elevation, topogra- serve as a flow pathway for energy, ma- phy, soils, hydrology, vegetation, and terials, and organisms. A stream corri- human uses. In terms of planning dor is above all a conduit that was restoration measures, corridor width is formed by and for collecting and trans- especially important for wildlife. When porting water and sediment. In addi- planning for maintenance of a given tion, many other types of materials and wildlife species, for example, the dimen- biota move throughout the system. sion and shape of the corridor must be wide enough to include enough suit- The stream corridor can function as a able habitat that this species can popu- conduit laterally, as well as longitudi- late the stream corridor. Corridors that nally, with movement by organisms and are too narrow may provide as much of materials in any number of directions. a barrier to some species’ movement as Materials or animals may further move would a complete gap in the corridor. across the stream corridor, from one side to another. Birds or small mam- On local scales, large woody debris that mals, for example, may cross a stream becomes lodged in the stream channel with a closed canopy by moving can create morphological changes to through its vegetation. Organic debris the stream and adjacent streambanks. and nutrients may fall from higher to

2–82 Chapter 2: Stream Corridor Processes, Characteristics, and Functions lower floodplains and into the stream isms) are in part the excesses of energy within corridors, affecting the food sup- from its own system. ply for stream invertebrates and fishes. One of the best known and studied ex- Moving material is important because it amples of aquatic species movement impacts the hydrology, habitat, and and interaction with the watershed is structure of the stream as well as the ter- the migration of salmon upstream for restrial habitat and connections in the spawning. After maturing in the ocean, floodplain and uplands. The structural the fish are dependent on access to attributes of connectivity and width also their upstream spawning grounds. In influence the conduit function. the case of Pacific salmon species, the For migratory or highly mobile wildlife, stream corridor is dependent upon the corridors serve as habitat and conduit resultant biomass and nutrient input of simultaneously. Corridors in combina- abundant spawning and dying adults tion with other suitable habitats, for ex- into the upper reaches of stream sys- ample, make it possible for songbirds tems during spawning. Thus, connectiv- to move from wintering habitat in the ity is often critical for aquatic species neo-tropics to northern, summer habi- transport, and in turn, nutrient trans- tats. Many species of birds can only fly port upstream from ocean waters to for limited distances before they must stream headwaters. rest and refuel. For stream corridors to Streams are also conduits for distribu- function effectively as conduits for these tion of plants and their establishment birds, they must be sufficiently con- in new areas (Malanson 1993). Flowing nected and be wide enough to provide water may transport and deposit seeds required migratory habitat. over considerable distances. In flood Stream corridors are also conduits for stage, mature plants may be uprooted, the movement of energy, which occurs relocated, and redeposited alive in new in many forms. The gravity-driven en- locations. Wildlife also help redistribute ergy of stream flow continually sculpts plants by ingesting and transporting and modifies the landscape. The corri- seeds throughout different parts of the dor modifies heat and energy from sun- corridor. light as it remains cooler in spring and Sediment (bed load or suspended load) summer and warmer in the fall. Stream is also transported through the stream. valleys are effective airsheds, moving Alluvial streams are dependent on the cool air from higher to lower elevations continual supply and transport of sedi- in the evening. The highly productive ment, but many of their fish and inver- plant communities of a corridor accu- tebrates can also be harmed by too mulate energy as living plant material, much fine sediment. When conditions and export large amounts in the form are altered, a stream may become either of leaf fall or detritus. The high levels starved of sediment or choked with sed- of primary productivity, nutrient flow, iment down-gradient. Streams lacking and leaf litter fall also fuel increased appropriate amounts of sediment at- decomposition in the corridor, allow- tempt to reestablish equilibrium through ing new transformations of energy and , bank erosion, and channel materials. At its outlet, a stream’s out- erosion. An appropriately structured puts to the next larger water body (e.g., stream corridor will optimize timing increased water volume, higher temper- and supply of sediment to the stream to ature, sediments, nutrients, and organ- improve sediment transport functions.

Functions and Dynamic Equilibrium 2–83 Local areas in the corridor are depen- nectivity (gap frequency) and corridor dent on the flow of materials from one width (Figure 2.40). Elements which point to another. In the salmonid ex- are moving along a stream corridor edge ample, the local upland area adjacent to may also be selectively filtered as they spawning grounds is dependent upon enter the stream corridor. In these cir- the nutrient transfer from the biomass cumstances it is the shape of the edge, of the fish into other terrestrial wildlife whether it is straight or convoluted, and off into the uplands. The local which has the greatest effect on filtering structure of the streambed and aquatic functions. Still, it is most often move- ecosystem are dependent upon the sedi- ment perpendicular to the stream corri- ment and woody material from up- dor which is most effectively filtered or stream and upslope to create a halted. self-regulating and stable channel. Materials may be transported, filtered, Stream corridor width is important or stopped altogether depending upon where the upland is frequently a sup- the width and connectedness of a plier of much of the natural load of stream corridor. Material movement sediment and biomass into the stream. across landscapes toward large river val- A wide, contiguous corridor acts as a leys may be intercepted and filtered by large conduit, allowing flow laterally stream corridors. Attributes such as the and longitudinally along the corridor. structure of native plant communities Conduit functions are often more lim- can physically affect the amount of ited in narrow or fragmented corridors. runoff entering a stream system through uptake, absorption, and interruption. Filter and Barrier Functions Vegetation in the corridor can filter out much of the overland flow of nutrients, sediment, and water. Siltation in larger streams can be re- duced through a network of stream cor- ridors functioning to filter excessive sediment. Stream corridors filter many of the upland materials from moving unimpeded across the landscape. Stream corridors may serve as barriers Ground water and surface water flows that prevent movement or filters that are filtered by plant parts below and allow selective penetration of energy, above ground. Chemical elements are materials and organisms. In many ways, intercepted by flora and fauna within the entire stream corridor serves benefi- stream corridors. A wider corridor pro- cially as a filter or barrier that reduces vides more effective filtering, and a con- , minimizes sedi- tiguous corridor functions as a filter ment transport, and often provides a along its entire length. natural boundary to land uses, plant communities, and some less mobile Breaks in a stream corridor can some- wildlife species. times have the effect of funneling dam- aging processes into that area. For Materials, energy, and organisms which example, a gap in contiguous vegetation moved into and through the stream cor- along a stream corridor can reduce the ridor may be filtered by structural attrib- filtering function by focusing increased utes of the corridor. Attributes affecting runoff into the area, leading to erosion, barrier and filter functions include con-

2–84 Chapter 2: Stream Corridor Processes, Characteristics, and Functions no vegetative buffer narrow vegetative buffer wide vegetative buffer dissolved substances

Figure 2.40: The width of the vegetation buffer influences filter and barrier functions. Dissolved substances, such as nitrogen, phosphorus, and other nutrients, entering a vegetated stream corridor are restricted from entering the channel by friction, root absorption, clay, and soil organic matter. Adapted from Ecology of Greenways: Design and Function of Linear Conservation Areas. Edited by Smith and Hellmund. © University of Minnesota Press 1993. gullying, and the free flow of sediments the conditions of the corridor may ger- and nutrients into the stream. minate and establish a population. The Edges at the boundaries of stream corri- lobes have acted as a selective filter col- dors begin the process of filtering. lecting some seeds at the edge and al- Abrupt edges concentrate initial filter- lowing other species to interact at the ing functions into a narrow area. A boundary (Forman 1995). gradual edge increases filtering and spreads it across a wider ecological gradient (Figure 2.41). Movement parallel to the corridor is affected by coves and lobes of an un- even corridor’s edge. These act as barri- ers or filters for materials flowing into the corridor. Individual plants may selectively capture materials such as wind-borne sediment, carbon, or pro- (a) (b) pagules as they pass through a convo- Figure 2.41: Edges can be (a) abrupt or luted edge. Herbivores traveling along (b) gradual. Abrupt edges, usually caused a boundary edge, for example, may stop by disturbances, tend to discourage movement between ecosystems and promote movement to rest and selectively feed in a shel- along the boundary. Gradual edges usually tered nook. The wind blows a few seeds occur in natural settings, are more diverse, into the corridor, and those suited to and encourage movement between ecosystems.

Functions and Dynamic Equilibrium 2–85 Source and Sink Functions they lack a suitable body of research and practical application guidelines. Forman (1995) offers three source and sink functions resulting from floodplain vegetation:

Decreased downstream flooding through floodwater moderation and/or uptake

Sources provide organisms, energy or Containment of sediments and materials to the surrounding landscape. other materials during flood stage Areas that function as sinks absorb or- Source of soil organic matter and ganisms, energy, or materials from the water-borne organic matter surrounding landscape. Influent and ef- Biotic and genetic source/sink relation- fluent reaches, discussed in Section 1.B ships can be complex. Interior forest of Chapter 1, are classic examples of birds are vulnerable to nest parasitism sources and sinks. The influent or “los- by cowbirds when they try to nest in ing” reach is a source of water to the too small a forest patch. For these aquifer, and the effluent or “gaining” species, small forest patches can be reach is a sink for ground water. considered sinks that reduce their pop- Stream corridors or features within them ulation numbers and genetic diversity can act as a source or a sink of environ- by causing failed reproduction. Large mental materials. Some stream corridors forest patches with sufficient interior act as both, depending on the time of habitat, in comparison, support success- year or location in the corridor. Stream- ful reproduction and serve as sources of banks most often act as a source, for more individuals and new genetic com- example, of sediment to the stream. At binations. times, however, they can function as sinks while flooding deposits new sedi- Dynamic Equilibrium ments there. At the landscape scale, cor- The first two chapters of this document ridors are connectors to various other have emphasized that, although stream patches of habitats in the landscape and corridors display consistent patterns in In constantly as such they are sources and conduits of their structure, processes, and functions, changing genetic material throughout the land- these patterns change naturally and con- ecosystems scape. like stream cor- stantly, even in the absence of human ridors, stability Stream corridors can also act as a sink disturbance. Despite frequent change, is the ability of for storage of surface water, ground streams and their corridors exhibita a system to water, nutrients, energy, and sediment dynamic form of stability. In constantly persist within allowing for materials to be temporarily changing ecosystems like stream corri- a range of con- fixed in the corridor. Dissolved sub- dors, stability is the ability of a system ditions. This stances, such as nitrogen, phosphorus, to persist within a range of conditions. phenomenon and other nutrients, entering a vege- This phenomenon is referred to as is referred to tated stream corridor are restricted from dynamic equilibrium. as dynamic entering the channel by friction, root equilibrium. The maintenance of dynamic equilib- absorption, clay, and soil organic mat- rium requires that a series of self-cor- ter. Although these functions of source recting mechanisms be active in the and sink are conceptually understood, stream corridor ecosystem. These mech-

2–86 Chapter 2: Stream Corridor Processes, Characteristics, and Functions anisms allow the ecosystem to control external stresses or disturbances within a certain range of responses thereby maintaining a self-sustaining condition. The threshold levels associated with Stability, as a characteristic of ecosystems, combines these ranges are difficult to identify and the concepts of resistance, resilience, and recovery. quantify. If they are exceeded, the sys- Resistance is the ability to maintain original form and tem can become unstable. Corridors functions. Resilience is the rate at which a system returns may then undergo a series of adjust- to a stable condition after a disturbance. Recovery is the ments to achieve a new steady state degree to which a system returns to its original condition condition, but usually after a long pe- after a disturbance. Natural systems have developed riod of time has elapsed. ways of coping with disturbance, in order to produce Many stream systems can accommodate recovery and stability. Human activities often superim- fairly significant disturbances and still pose additional disturbances which may exceed the return to functional condition in a rea- recovery capability of a natural system. The fact that sonable time frame, once the source of change occurs, however, does not always mean a system the disturbance is controlled or re- is unstable or in poor condition. moved. Passive restoration is based on The term mosaic stability is used to denote the stability this tendency of ecosystems to heal of a larger system within which local changes still take themselves when external stresses are place. Mosaic stability, or the lack thereof, illustrates the removed. Often the removal of stress importance of the landscape perspective in making site- and the time to recover naturally are an specific decisions. For example, in a rapidly urbanizing economical and effective restoration landscape, a riparian system denuded by a 100-year strategy. When significant disturbance flood may represent a harmful break in already dimin- and alteration has occurred, however, a ished habitat that splits and isolates populations of a stream corridor may require several rare amphibian species. In contrast, the same riparian decades to restore itself. Even then, the system undergoing flooding in a less-developed land- recovered system may be a very differ- scape may not be a geographic barrier to the amphibian, ent type of stream that, although at but merely the mosaic of constantly shifting suitable and equilibrium again, is of severely dimin- unsuitable habitats in an unconfined, naturally function- ished ecological value in comparison ing stream. The latter landscape with mosaic stability is with its previous potential. When not likely to need restoration while the former landscape restoration practitioners’ analysis indi- without mosaic stability is likely to need it urgently. cates lengthy recovery time or dubious Successful restoration of any stream corridor requires an recovery potential for a stream, they understanding of these key underlying concepts. may decide to use active restoration techniques to reestablish a more func- tional channel form, corridor structure, and biological community in a much shorter time frame. The main benefit of an active restoration approach is regain- turbance. In addition, disturbances can ing functionality more quickly, but the often stress the system beyond its nat- biggest challenge is to plan, design, and ural ability to recover. In these instances implement correctly to reestablish the restoration is needed to remove the desired state of dynamic equilibrium. cause of the disturbance or stress (pas- This new equilibrium condition, how- sive) or to repair damages to the struc- ever, may not be the same that existed ture and functions of the stream prior to the initial occurrence of the dis- corridor ecosystem (active).

Functions and Dynamic Equilibrium 2–87