2.A Hydrologic and Hydraulic Processes • Where does stream flow come from? • What processes affect or are involved with stream flow? • How fast, how much, how deep, how often and when does water flow? • How is hydrology different in urban stream corridors? 2.B Geomorphic Processes • What factors affect the channel cross section and channel profile? • How are water and sediment related? 2 • Where does sediment come from and how is it transported downstream? • What is an equilibrium channel? • What should a channel look like in cross section and in profile? • How do channel adjustments occur? • What is a floodplain? • Is there an important relationship between a stream and its floodplain? 2.C Chemical Processes • What are the major chemical constituents of water? • What are some important relationships between physical habitat and key chemical parameters? • How are the chemical and physical parameters critical to the aquatic life in a stream corridor? • What are the natural chemical processes in a stream corridor and water column? • How do disturbances in the stream corridor affect the chemical characteristics of stream water? 2.D Biological Processes • What are the important biological components of a stream corridor? • What biological activities and organisms can be found within a stream corridor? • How does the structure of stream corridors support various populations of organisms? • What are the structural features of aquatic systems that contribute to the biological diversity of stream corridors? • What are some important biological processes that occur within a stream corridor? • What role do fish have in stream corridor restoration? 2.E Stream Corridor Functions and Dynamic Equilibrium • What are the major ecological functions of stream corridors? • How are these ecological functions maintained over time? • Is a stream corridor stable? • Are these functions related? • How does a stream corridor respond to all the natural forces acting on it (i.e., dynamic equilibrium)? 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 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 infiltration 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. I 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 I Leaf texture. Rough leaves store more concentrates on water as it moves in the water than smooth leaves. longitudinal dimension, specifically as I 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 I 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. I Interception, transpiration, and I Age of the plant community. Some evapotranspiration. vegetative stands become more dense I Infiltration, soil moisture, and with age; others become less dense. ground water. The intensity, duration, and frequency I 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.