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According to the World Commission on Dams, 46% of the 106 primary watersheds on earth are affected by dams. These effects can include temperature changes (water held in a reservoir warms, while water which is released over the dam's head is cooled) and dissolved oxygen level changes (the warmer water in a dam's reservoir will have lower dissolved oxygen levels resulting from higher water temperatures and slower water velocity, while water below the dam may become super-saturated with oxygen and poison fish). These changes often favor invasive species, which can then outcompete the native biota. Dams also change the natural flow regimes, which are important triggers for biological cycles. Flow levels can enhance or suppress reproductive success for many species, as well serving to redistribute substrates (material comprising the river bottom) and bed-loads (large particles carried along the bottom) (Young, 1997). Furthermore, starvation of sediments because of retention by dams can alter the substrate composition downstream with huge effects on fish; studies on the Colorado River indicated that natural reproduction of fish species was suppressed because sandbar formation had ceased due to a lack of sediments (Young, 1997). The WCD reports that in many cases wetlands dry out and recharge of groundwater is diminished. Besides "trapping" water behind them, dams also act as particle traps, holding back nutrients and sediment. The downstream ecosystems that rely on these nutrients can suffer severely; the crash of Kokanee salmon was attributed to the drastic decrease in nutrient loading caused by the construction of two dams (Wuest). The changes in sediment transport can heavily influence the channel, floodplain, and delta morphology. In coastal areas, the erosion caused by waves is no longer counteracted by deposition of sediment; the WCD reports that the coastline of Togo and Benin has decreased by 10-15 meters per year after the Akosombo Dam on the Volta River was completed. There are indications that erosion may also result in a lack of floodplain fertility.
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