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River Services: A Snapshot of River Partners New Science Webpages

Editor’s Note:

Over the last several years, nonstructural flood management approaches have been tested within the Sacramento-San Joaquin river system. These onthe ground projects combine innovative habitat restoration components with set-back levees and floodwater storage projects. River Partners’ work on the San Joaquin River National Wildlife Refuge (see Irv Schiffman’s article, A Tour of a Successful Restoration Project, pg. 2) is an example of these projects. In essence, by widening the floodway, they attempt to restore some of the river services explained below. However, the extent to which river services can be enhanced will always be limited by their regulation by dams and levees. (References are listed on our website at www.riverpartners.org.)

Prior to extensive human settlement and development along the floodplains of rivers, rivers were unregulated – free of dams, levees, straightening and armoring. The flows through rivers were determined by climate and precipitation. When flooding occurred, the floodplains received the extra water. Healthy rivers and floodplains sculpted the landscape and nourished the plants and wildlife dependent on riparian ecosystems. These healthy rivers and floodplains also provided many services that were beneficial to people, which encouraged human settlement and development of the floodplains. Regulation of the rivers allowed protection from the effects of floods, and improved use of the rivers for transportation and water supply for agriculture and urban uses. Through regulation, many human services have been developed, but at the same time, countless ecological benefits have been reduced or lost all together.

Services Provided by Unregulated Rivers

Storage of Floodwaters

The floodplain provides the open space along river channels across which high river flows can spread out. Excess water that cannot be held by the river channel after a precipitation event can be stored on the floodplain. These excess flows eventually return to the river channel, either as surface flows or groundwater discharge. The floodplains reduce the magnitude of flooding and potential for flood damage to surrounding areas.

Groundwater Recharge

Water that enters the floodplain may percolate through the soil, or eventually return to the river channel as surface water. The topography of floodplains, and the presence of vegetation, can allow floodwaters to pond, giving the water time to slowly penetrate the ground and recharge aquifers. Ground water is an important reserve that can replenish the river channel during low flow events, which can help prevent flows from becoming too low to adequately support aquatic ecosystems.

Water Purification

Some of the floodwaters that move across the floodplain percolate through the vegetation and soils. This process helps purify the water that eventually makes its way into the aquifer. Ground water is one main supply of municipal drinking water. Vegetation helps filter water and traps sediment from river flows entering the floodplain. The vegetation also filters waters returning to the river channel, and therefore reduces the amount of debris and sediment that is released back into the river.

The ability of floodplains to filter water has significant impacts on the quality of water that enters the river and ground water aquifers. The floodplain acts as a buffer between surface runoff from surrounding land uses and the main river channel. For example, floodplains, especially where vegetated, can improve the quality of water from agricultural runoff by trapping nutrients and impurities.

Floodplain Building

River flows are capable of carrying sediment; faster flows can move larger sediment. When water moves across the floodplain and slows, sediment is deposited. This process adds nutrient rich sediments across the floodplain, which increases soil fertility and builds the floodplains.

Figure I: Illustration of how flexible stem plants react to flood waters.

Erosion Control

Riparian vegetation is a critical component of a healthy river ecosystem. In addition to its role in water purification, vegetated floodplains can help stabilize stream banks and floodplain soils during high water events. The roots of plants structurally support soils, acting like sections of rebar used to help bind cement. The above ground plant material also absorbs much of the impact of water, slowing the water that passes over the soil.

Native Wildlife

Riparian forests in California support a greater diversity of wildlife than any other habitat type. Riparian vegetation along river channels also functions as primary regional migration routes for most wildlife. Over 225 species of birds, mammals, reptiles and amphibians are dependent on California’s riparian habitats. Special status mammals documented using restored riparian habitat in the San Joaquin Valley include the Riparian Brush rabbit, and along the Sacramento River Western mastiff bats, Pallid bats, Western red bats, and Yuma myotis. (Golet et al 2008). Birds are the most diverse and most studied of the wildlife in the riparian zone. The types of species that riparian vegetation supports range from Swainson’s Hawks that nest in tall cottonwood or valley oak trees, to House Wrens that forage on the floor of the forest and inside debris piles.

Salmon

Before intensive human settlement of rivers, the creation of dams and mining operations, the Sacramento/San Joaquin river system supported booming salmon (Oncorhynchus spp.) populations, with up to 3 million salmon returning every year. Like the native plants, salmon adapted to the unpredictability of rivers and environmental conditions. Chinook salmon (Oncorhynchus tshawytscha) evolved into different races that divided the river resources temporally. These runs of salmon returned to the rivers in different seasons, thus reducing competition but also increasing the overall life-history diversity of the species.

Such diversity provided Chinook salmon with a buffer against their often changing environment. Certain years favored the lifehistories of the different runs. Even among runs there was a high degree of genetic variability, which further increased chances of survival amidst the ever changing conditions.

Salmon were able to diversify in part because of the variation in spawning and rearing habitats that the tributaries and main stems of the Sacramento/San Joaquin system provided. High elevation and low elevation streams, rapidly moving waters and pools, cool water temperatures and warmer rearing grounds, instream and floodplain habitats were accessible to the salmon. Also present in the varied habitats was the appropriate spawning gravel substrate, which is not muddled by finer sediments like sand. Unregulated rivers are able to carry and sort sediment, depositing clean gravel that can be used for spawning.

Native Pollinators

Due to the variety of native plants in riparian areas, there is a great diversity of pollinating insects. Native pollinators are valuable resources to agricultural areas, especially organic farms adjacent to riparian plant communities (Kremen and others 2002). With the decline of honey bee populations, native pollinators are increasingly considered viable alternatives to meet the pollination demands of agriculture.

Carbon Sequestration

Carbon sequestration occurs when plants absorb carbon dioxide from the atmosphere for photosynthesis, store the carbon and release the oxygen. Carbon can be stored in above and below ground biomass of plants, in dead plant material and in soil. In general, the rate of carbon sequestration increases as the plants age (Giese and others 2003), but eventually mature plants will slow their rate of sequestration (EPA site). Riparian plant communities sequester carbon at higher rates than many other plant community types. Due to the dynamic nature of river systems, which are constantly eroding established forests and rebuilding point bars where young forests rapidly develop, there are multiple ages of plant communities that continue to grow and sequester carbon at a faster rate.

Recreation and Aesthetics

Healthy rivers and floodplains contain a variety of native vegetation, multiple wildlife habitats, and clean water. This formula creates excellent outdoor recreation such as hunting, fishing, hiking, camping, swimming, and boating.

The above article originally appeared in the December 2009 issue of the River Partners Journal.