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 Healthy (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 can not 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.
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.
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 trap 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.
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.
Meandering portion of the Sacramento River near the Pine Creek unit of the Sacramento River National Wildlife Refuge. Photo by Tom Griggs.
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.
The riparian zone is characterized by a unique set of physical ecological factors in comparison to the surrounding regional landscape (Gregory et al. 1991). These factors include flooding by the river, rich and productive soils, a water table that is within reach of plant roots, and species of plants and wildlife that are adapted to the timing of fluvial events such as flooding, drought, sediment transport and channel movement. This dynamic habitat creates a wide variety of growing conditions for riparian plants, and over time they develop into various structural forms (forests, woodlands, shrublands, meadows and grasslands) across the floodplain. The heterogeneity of riparian forests creates numerous habitat features that explain why riparian forests in California support a greater diversity of wildlife than any other habitat type (Smith 1980). 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 (RHJV 2004). Most terrestrial mammals found in California spend time in (or require) riparian areas. Common low-elevation mammal species include raccoon, striped skunk, opossum, coyote, and black-tailed deer. Where large cavities exist in old, large trees, ringtail cats can be locally abundant. Rodent species that rely on riparian vegetation are few: beaver and gray squirrel. Ground squirrels, pocket gophers, and meadow voles live only around the margins of riparian areas where woody vegetation is sparse or non-existent. 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.
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 life-histories 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, in-stream 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. California's freshwater recreational fishery generates $4 billion and almost 27,000 jobs (Water4Fish).
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 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. Riparian areas provide open spaces for parks and educational exhibits. Wild rivers and riparian forests offer places to explore and escape development. Mosaics of plant communities surrounding meandering rivers provide scenic landscapes.
The Benefits of Regulated Rivers
In the 1900s, local, state and federal governments began cooperating in the development and management of flood control systems for California. The US Army Corps of Engineers oversaw the implementation of over 1600 miles of levees throughout the Central Valley. Today in California, the Flood Board regulates construction and maintenance of the levees and the 1300 miles of floodways designated for flood discharge. Development of the floodplains continues to increase each year, with commercial, residential and agricultural growth. More than $47 billion in infrastructure is protected by the Central Valley levees (DWR levee website).
California is essentially arid to semi-arid throughout much of the state, with a Mediterranean climate producing most precipitation in the winter and spring and drought in the summer and fall, during the growth season. To provide a year round source of water to the state for irrigation and drinking water, Federal legislation was passed to create the Central Valley Project (CVP) in 1933. Almost 200 reservoirs each have the capacity to store greater than 10,000 acre-feet of river flows, and an extensive plumbing system delivers the water throughout the state (CA water plan 2005). Some of the biggest agriculture operations and cities in the US depend on water from these reservoirs. Eighty percent of the water stored in California's reservoirs supplies agriculture, which as an industry supplies 50% of the nation's fruit and vegetables. California agriculture exported $10.9 billion in 2007 (California Food and Agriculture website). Initially the Federal Government invested approximately $3 billion into the CVP, and it is estimated to have returned $300 billion in growth of agriculture and related industries.
The Costs of River Regulation
Los Angeles River in White Oak, California.
River regulation allows a greater portion of the floodplain and water supply to be accessible for other uses, and contributes significantly to the State's economy. While people have gained certain services through river regulation, many other services that healthy rivers and floodplains can provide have been impaired by regulation. Unfortunately, the services people gain from river regulation come at a cost of continued repair, maintenance and intermittent failure, while healthy river services are free. (In 2006, Governor Schwarzenegger commissioned $500 million state funds for levee repair projects.) The indirect costs that result from the impaired riparian ecosystems are harder to quantify.
Since the implementation of the CVP and levee system, the amount of riparian forests and wetlands in the Central Valley has declined by more that 90%. Loss of habitat is the biggest threat to terrestrial wildlife that depend on riparian areas. Landbird population declines in western North America are attributed primarily to the loss of riparian habitat (DeSante and George 1994).
Levees restrict the area of the floodplain that can provide river services. With levees in place, less of the area becomes covered in water, so there is a smaller volume of water that can percolate through vegetation and soils and drain into the water table. Native plants along the floodplain cut off from the river by levees lose the ability to establish once the floodplain is no longer replenished by flood flows. Floodplains that lack vegetation can not effectively purify water or reduce erosion. Additionally, with a reduced floodplain there are fewer native plants to support wildlife habitat and native pollinators. Dams cut off the upstream tributaries available to anadromous fish, which reduces their potential habitat by 95%. Moreover, dams reduce the amount and quality of gravel used by fish for spawning. Poor returns of salmon into the Central Valley rivers caused the commercial fish industry to be shut down for 2008 and 2009.
Dams and levees alter the hydrology and sediment transport of rivers. Levees reduce the space across which floodwaters can spread, which causes the rivers to be unnaturally deep. The addition of dams exacerbates this issue. Dams trap coarse sediment, and release waters with a reduced sediment load that are "hungry" to meet their sediment capacities (Kondolf 1994). Higher, sediment hungry flows will cause significant erosion, leading to either deeply incised or wider channels. These hungry waters can lead to channel incision downstream of the dam, resulting in bank erosion that can threaten infrastructure. Floodplains can not be rebuilt through sediment transport with levees in place.
Levees encourage development of the floodplains due to the perceived protection from flooding. However, a 1955 flood resulted in levee failure, the death of 74 people and over $200 in economic losses (Harding 1960). Another major flood in 1986 also caused levee failure, 13 deaths and $400 million in damages (Progar 2008). Back to back flooding in 1995 and 1997 caused over $4 billion in damages (DWR website).
Increased development along floodplains increases the waste products that are disposed into the rivers. The input of excess nutrients and unnatural compounds into rivers can affect aquatic life. The Delta is particularly sensitive to water quality, because it is a nursery ground for many aquatic species.
The entire river system within a watershed is connected; therefore the placement of dams or levees will affect the waterways both downstream and upstream of the regulation. Native plants and wildlife adapted to unregulated rivers typically suffer in the presence of dams or levees, resulting in a loss of multiple river services spread throughout the watershed.