Technology is not the only solution to securing the future of our water resources. Restoring aquatic ecosystems can provide many benefits, such as purifying water and reducing erosion (Figure 6.34). The first step to restoring an aquatic ecosystem is ensuring that there is sufficient water to support populations of aquatic species. Many states have laws requiring that some minimum instream flow be maintained in rivers. In other cases, river managers have gone beyond minimum flows and have restored some of the historical floods to approximate a natural flow regime.

Historically, the Colorado River flooded each spring as snow melted in its headwaters in the Rocky Mountains. These torrential flows also carried a large amount of sediment that was deposited in sandbars along the length of the river (Figure 6.35). Sandbars are especially important in the Grand Canyon, where the quiet backwater areas they create serve as critical nursery areas for the young of endangered native fish. Sandbars themselves provide camping and pullout places for rafters and backpackers. The size of the spring flows was greatly reduced, however, as dams were built on the river system. Damming the Colorado River also reduced the amount of sediment available in the river for forming sandbars, since reservoirs trap sediment.

The progressive taming of the river was largely done in 1963, with the completion of Glen Canyon Dam 24 kilometers (15 miles) upriver from the Grand Canyon. The reservoir behind the dam, Lake Powell, extends nearly 300 kilometers (186 miles) along its main channel and stores 30 km³ of water when full (Figure 6.36). In the absence of flooding and greatly reduced sediment below Lake Powell, sandbars were being steadily lost in the Grand Canyon. Colonization by invasive plants, especially saltcedar, Tamarix species (see Figure 4.28, page 119), has converted some sandbars into dense thickets of vegetation, while erosion gradually washed away others. The result was reduced quality of habitat for both native fish and recreational rafters.

River managers proposed controlled flooding to help sustain sandbars in the Colorado River. Their plan was to use short pulses of high river flows to suspend sediments on the riverbed, which would settle out, forming sandbars as river flow receded. The first controlled flood below Glen Canyon Dam took place in the spring of 1996, with others following in 2004, 2008, and 2012. During controlled flooding, river managers leave the floodgates open for about 3 to 5 days and time the floods to follow inputs of sand from tributaries of the Colorado River. For example, the Paria River, which flows into the Colorado River downstream from Glen Canyon Dam, washed more than 1 million tons of sediment into the river a few months before the 2004 controlled flood. Controlled flooding has been successful at restoring sandbars to the Colorado River shoreline within the Grand Canyon (Figure 6.37).

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The Kissimmee River in central Florida was altered dramatically in the 1940s during efforts to control flooding on private property. Flood control efforts converted the meandering channel to a straight, 90-kilometer-long canal, which was 9 meters (30 feet) deep by 100 meters (328 feet) wide (Figure 6.38a). Because the canal was completely cut off from the floodplain, the exchange of nutrients and organic matter between the river and floodplain was eliminated. The impact on biodiversity was dramatic. For instance, the number of waterfowl declined by 90% and the number of nesting bald eagles fell by 75%. In addition, the canal became a sink for organic matter and dissolved oxygen levels (the amount of oxygen molecules in the water) fell, leading to the decline of a once thriving sport fishery for largemouth bass, which require higher oxygen levels.

The Kissimmee restoration project, which began in 1992, is the largest ecosystem restoration attempted to date. When completed in 2015, the project will have restored 100 km² (40 mi²) of river–floodplain ecosystem, containing over 8,000 hectares (20,000 acres) of productive wetlands. Restoration goals also include reestablishing 61 kilometers (40 miles) of meandering river channel (Figure 6.38b).

The response of the biological community to restoration has been rapid and impressive. Numbers of wading birds in the restored river and floodplain areas have increased five-fold. Numerous duck and shorebird species that had been absent from the landscape for decades have returned. Organic deposits in the riverbed have been reduced by over 70% and dissolved oxygen levels have increased. Those increases in dissolved oxygen and habitat complexity have been accompanied by a resurgence of populations of largemouth bass and other sunfish.

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