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? stream 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 streams but is affected by the kinds Characteristics of soils and alluvial features within The fish, wildlife, plants, and hu- the channel, in the floodplain, and mans that use, live in, or just visit in the uplands. The amount and the stream corridor are key ele- kind of sediments 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 sediment 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 surface water 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 rivers that originate in high in mid-latitude areas with colder sea- mountain areas and for continental re- sonal temperatures.
cloud formation
rain clouds evaporation
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m s precipitation e o o v r r n f f m a r
o t r f
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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 surface runoff. 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: