Physical River Processes
The area, elevation, and geology of the channel in a watershed determine the nature of river flows, sediment transport and channel meander. These characteristics of the watershed also shape the riparian vegetation within its floodplain, by determining the plant species that will survive and the vegetation structure that is formed.
A watershed is the entire land area that drains a particular river system. It is characterized by its area, elevation and geology. Watersheds that include higher elevation rivers will receive snow melt in the late spring, early summer.
Watershed Characteristics Affect River Flows
Watersheds with large areas have the potential to generate large river flows because they have an increased capacity to store water. The velocity of the flows depends on the geology of the watershed, and resulting slope of the channel, which varies among the reaches of the streams throughout the watershed. Assisted by the force of gravity, rivers that flow through steep reaches of a watershed have the potential to move at much higher velocities than rivers that flow through low gradient channels.
Elevation of the watershed can affect the timing of high flows through the river. High elevation watersheds receive abundant snow during the winter which will melt and increase river flows during the late spring/early summer. For example, the watersheds of many rivers in the San Joaquin Valley are set at higher elevations than rivers of the Sacramento Valley. As snow in the high elevation watershed melts, the flow in the San Joaquin River increases, often producing floods relatively late in the water-year. Compare this to the snowmelt runoff in Sacramento, which is much less, and causes relatively minor flow increases during the spring and summer.
Watershed Characteristics Affect Sediment Load
The characteristics of a watershed that affect river flows – area, elevation and geology - also influence the amount and rate of sediment that can be carried by the river. The sediment load that a river carries is the result of geologic erosion of its watershed. The force of flowing water varies depending on the volume of water in the watershed and the channel slope. Steep gradient rivers have more erosive power than low gradient rivers and may be deeply incised into the surrounding landscape and adjacent floodplain areas. Low gradient rivers are often depositional with large broad floodplains.
Larger, faster flows can carry heavier sediment farther than slower flows. The changes in flows over time distribute the sediments across the floodplain into layers of sediment shaped into a characteristic geomorphology. Water moves more quickly down a steep canyon than through a relatively flat valley floor. Rivers that flow through wide valleys tend to deposit sediment frequently during normal flows, because the sediment cannot be carried far by slow moving water. As a result, rivers in wide valleys continually build their floodplains. Rivers that flow through steep canyons are more erosive because of their increased velocity. Typically, these rivers form deep and narrow channels and because of the velocity of the water, the sediment is carried by the river flow until the water velocity decreases and sediments fall out of solution.
The most important results of the sediment transport process are bank erosion and point-bar formation, which overtime builds floodplains by deposition of sediment. Together, bank erosion, point-bar formation and floodplain building result in the lateral movement of the channel, or channel meander.
Some properties of channel meander exhibited by the Sacramento River. Where streams flow over low gradients through erodable banks, the velocity of the water causes the channel to meander. Erosion occurs on the outerbends where water moves fastest, and sediment is deposited on the inner bends (where water velocity is low) and forms point bars. If a meander bend is cut off from the rest of the channel, an oxbow lake is formed. Images adopted from Earth Science Australia.
Floodplains Are Built by Rivers
Rivers transport sediment as they meander through the geologic constraints of the landscape. The sediment is deposited in layers overtime that build floodplains along the river channel. Floodplains typically have multiple terraces that increase in elevation away from the river channel. The higher elevation terraces have had sediment deposited for a longer period of time than the lands immediately adjacent to the river. The river channel can meander through an older section of the floodplain and erode the sediment, resulting in a diversity of floodplain elevations, especially if the river is free to meander.
Floodplains receive the high river flows that can no longer be contained by the channel. The higher elevation floodplains require larger flows in order to be flooded; this is why floodplains are sometimes classified as 50 to 200 year floodplains. The 200 year floodplains are higher elevation than the 50 year floodplains, and therefore the 200 floodplain floods less frequently (once every 200 years) than the 50 year floodplain (once every 50 years).
Floodplains differ by the layering of sediments (soil texture and elevation) and the frequency with which they flood. Plants are adapted to certain soil textures and hydrologic regimes, therefore, the plant communities are varied throughout the floodplain.
The floodplain of a river often has multiple elevations, each of which may have a different plant community because of the soil texture and frequency of flooding.
Plants and Animals Adapt to Variation in River Flows Over Time
Plants and animals adapt to a river’s natural variation in flow volumes overtime. Periodic flooding by the river is a fundamental characteristic of floodplain and riparian ecology. The frequency (recurrence interval) and duration of flood events over time shape the physical habitat and create the ecological restraints that determine the species composition and community structure on a site.
A hydrograph is a graphical display of average flow over a specified period of time. In other words, a hydrograph can be used to evaluate flow patterns in a day, over a year, or over several years. Most riparian species of plants and animals are adapted to the river’s hydrograph for reproduction, growth, and survival.
The natural hydrograph for rivers in California is an inverted U-shape, with peak flows in the winter and spring (November through June). The slowing or reduction in magnitude of flows during late spring and early summer, as rainfall tapers to nothing, is biologically important to most plants that grow in the riparian zone. Seed-release, seed dispersal, and seedling establishment are adaptations to the hydrograph by most riparian plants. Cottonwood is the most studied in this regard, although all species of willows have a similar behavior in response to the hydrograph.
Likewise most species of fish are adapted to the hydrograph. The entire freshwater phase of the salmonid life cycle is adapted to natural flow regimes and associated water temperatures, including adult upstream migration, spawning, juvenile rearing and out migration. Adult salmon require cold, deep holding pools and cool oxygen-rich waters flowing over and through spawning gravels. Juvenile salmon exhibit higher growth rates when they forage in the warmer shallow waters of inundated floodplains in the spring. Resident species such as the Sacramento splittail spawn on submerged floodplain vegetation during early spring floods.
This figure shows the natural hydrograph of the Trinity River in blue, overlaid by the lifecycles of two riparian trees, black cottonwood and narrowleaf willow, and the fall-run Chinook salmon. The figure shows how the timing of the salmon arrival, their spawning, hatching and juvenile growth all occur at characteristic times on the hydrograph. Likewise, cottonwood and narrowleaf willow seed release and seedling establishment rely upon the timing and magnitude of flows that are controlled by the hydrograph. Note that the natural hydrograph in blue represents flows before the dam was built, and exhibits high variability in flow, while the flows after dam construction shown in yellow exhibit very little variability and are in general very low all year long. Figure used with permission from McBain and Trush, Inc.
Human Modification of Physical River Processes
California’s Central Valley riparian areas have a long history of human use. Throughout the Central Valley levees were constructed to protect farmland from scour and sediment deposition during floods. The construction of dams for flood control and water supply started in the 1930s and continued into the 1970s, allowing most riparian lands to be converted to agriculture. Today, major dams block virtually all the large rivers in the Central Valley, with the resulting loss of 95 percent of pre-European acres of riparian habitat.
Riparian habitat originally extended for a few miles along the floodplains, but the vast majority has been converted to other land uses. This figure shows the extent of riparian habitat (shown in green) along the Middle Sacramento River, between Red Bluff and Colusa, historically and today. The blue sections represent the units of the Sacramento River National Wildlife Refuge, which conserves existing riparian forests and recreates additional riparian forests through restoration. Click here for more information about the Sacramento River NWR.
Riparian vegetation and wildlife are adapted to the physical river processes of flooding, sediment transport, and channel meander. River and floodplain management by humans through the use of dams, levees, bank stabilization, and water diversions significantly modifies the timing and magnitude of these processes.
These changes have altered the ecology of the river channels and floodplains to such a degree that many characteristic riparian species reproduce only on rare occasions. In addition, the structure of the vegetation has changed thereby eliminating habitat for many wildlife species, and allowing many non-native invasive species of plants to dominate the floodplain.
The flood control features along the Sacramento River are extensive. This figure shows the general flood control features along the Sacramento River, last updated in November of 2003. This map is used with permission from DWR; this and other maps can be found at http://www.water.ca.gov/floodmgmt/pubs/#
Dams for flood control and for water storage probably have the most significant ecological impact on floodplain biology. The dams have modified the major river processes, including the greatly altered seasonality of flows below the dams and the cut-off of sediment and organic matter transport. Dams impact the life histories of both plants and animals, resulting in many species being unable to survive or reproduce.
Dams severely modify the amount and timing of flows in the river below the dam. In addition, a dam usually reduces the magnitude of the high flow events that historically reshape and rejuvenate the channel through erosion and deposition of sediment.
Incoming sediment carried by the river from its watershed is trapped in the reservoir behind the dam. The resulting impact on the river below a dam is often a dramatic change in the quality of the sediments. The finer sediments (sand, silt, and clay) are washed downstream and only the coarser gravels and cobbles remain. Consequently, floodplain building may cease below the dam, yet channel and bank erosion may continue, resulting in entrenched channels that are much lower than the floodplain and flood it less frequently.
Dams block the transport of organic material, such as large wood and vegetation detritus. These materials provide nutrients, food, and shelter for aquatic life.
Levees that are constructed to protect riverside property from flooding effectively disconnect (or isolate) the river from its floodplain. The biological response to this isolation is ecological degradation of the plant and animal communities and the invasion of many weedy species that ordinarily would not be present due to flooding.
Bank stabilization is often accomplished by the use of rip-rap rock placed upon the bank from its toe to its crest in order to prevent bank erosion. In meandering systems, rock used in this way may halt natural river movements, effectively eliminating one form of natural sediment recruitment, and halting or impeding channel meander responsible for creating and rejuvenating plant and wildlife habitat. Levees or bank stabilization that extend for long distances on both sides of a channel (termed channelization) will cause hydraulic forces in the channel to be more intense/extreme due the increased depth of flows. This will result in increased rates of bank erosion and channel-scour, and the development of an entrenched channel.
Water diversions remove water from the river (or other body of water) to be used for other purposes, such as irrigation, industrial or domestic uses. Water diversions reduce the quantity of water in the downstream channel and greatly change water temperature, affecting river processes and hydrology. Water diversions include collection of surface and ground water. Ground water pumping, including conjunctive use programs, may affect local and regional water table depths, possibly affecting restoration project success because the local water table may drop below the rooting depth of vegetation. For more information about conjunctive use, see the California Department of Water Resources conjunctive use webpage.
Historic gold mining and modern gravel mining have resulted in extreme modification of in-stream and floodplain geomorphology. Large mining pits (covering many acres) are left behind after mining ends. These pits are unnaturally deep, they often capture the active channel, and they support non-native predatory fish (bass). In addition, the mining process literally turns the sediments upside-down; the channel and floodplain end up composed primarily of cobbles and gravel with most of the fine sediments (clay and silt) washing away during mining activities. Cobbles and gravel do not support plant growth.
Land-leveling for Agriculture
In the Central Valley most agriculture fields have been leveled. High water channels on the floodplain are filled and the natural drainage is altered. Land-leveling changes the local patterns of flood flows.
Conversion to Agriculture
Agriculture conversion physically replaces the complex, multi-layered riparian vegetation with a uniform vegetation pattern composed of one crop species. Most wildlife only use agricultural fields for movement to adjacent forest patches, or for seasonal uses such as foraging by waterfowl. Agriculture land cover typically cannot sustain wildlife populations because they do not provide enough cover types or food. Agricultural conversion can result in a highly fragmented (non-contiguous) riparian habitat. These remnants are usually too small to support the needs of wildlife. For example, the vegetation structure might be perfect for nesting for a focal bird species, but the number of acres is not large enough to support the insect food that the species requires to raise a brood. Agriculture often generates irrigation drain-water that finds its way into the river. This drain water can deliver pesticides and fertilizers into the river, changing aquatic communities and compromising water quality. Drain-water is typically a much higher temperature after it has flowed through a field and can have deleterious effects to local fish populations, depending upon the water volume into which it drains.
Livestock grazing impacts the watershed by affecting the timing of flows and the transport of sediments throughout the watershed. The livestock compact the ground, slowing percolation of water, and grazing shortens the vegetation. Compacted soils and reduced vegetation cause the velocity of water runoff to increase, which in turn causes more surface erosion in the watershed and adds abundant fine sediment to the river. Intensive grazing over many years in the riparian zone often results in a reduction of the cover and density of the understory, the deepening of the stream channel (entrenchment), and the consequent reduction in many species of wildlife that rely upon dense understory vegetation near open water. In recent years government land management agencies – Bureau of Land Management and the Natural Resources Conservation Service – have been actively fencing riparian areas to keep out the livestock.
Logging and the road-building required to support it can have major disruptive impacts upon a river and its watershed. Logging practices in the watershed usually results in an increase in fine sediment run-off that can fill the river channel. The geology of the Coast Ranges of California is especially susceptible to erosion after logging.
Urbanization along a river results in its channelization and typically reduction or removal of all riparian vegetation and an increase in impervious cover such as concrete and pavement. Impervious cover can result in increased run off and eliminates permeable ground where water can recharge underground aquifers. Where patches of riparian vegetation remain as parks, wildlife use is minimal because of the lack of proper vegetation structure, high density of human use, and feral animals, most usually domestic cats.