Issues in Ecology Number 10 Winter 2003 BOX 2-THE COLORADO RIVER The Colorado River is one of the most highly sed rive s in the world.Two reservoirs lakes Powell and Mead along with 12 other larne equations desianed to maximize both hydroelectric generation and water supplies for.domestic.and industrial use in seven states acre oss the Western lnited state s and Mexico.More than 30 million people depend on Colorado River water.The original Colorado River Compact of 198 llocated all water for societal use.(Actually it over-allocated because typical water volumes were overestimated while year-to-year variability was ignored.) Physical changes to the river below the dams have been profound.Flow in the Colorado River is snowmelt driven.and pre-dam flow patterns were dominated by large discharges from Apri vs in late summe and fall. The river carried tremendou rom nighly erodible Colorado Plateau, temperatures were melt,and peak ur in any m ases as gree ic me vital role rapped behin and the waters below are wate the bo ers of most reservoirs, ature or hundreds w th the natur 、the upper reversal of Colora re water is release Ecological responses to the dams have been equally profound.The clear,cold tail waters below the dams,in conjunction with widespread introduction of non-native species,have promoted food webs that are alien to the Colorado River.Prior to regulation,the organic matter that fueled the river food web primarily originated on land and was carried into the river during large runoft events.Now,organic matter is supplied largely by luxuriant mats of algae that grow on the bottom of the river.The algae are consumed by insects and other invertebrates that historically occurred only in the much colder tributanes or the Colorado;these insects and invertebrates are in turn eaten by non-native rainbow and brown trout.Below the Glen Canyon Dam that holds Lake Powell,only four out of eight indigenous fish species rema hes,many of which either compete witl or directly feed on the endangered native fish 00 ning because the trees are upstream res f annual floods prever are more tolerant of these modified conditions have The effects of 14 major dams and hundreds of water diversions have been felt all the way to the river mouth.Since completion of the Glen Canyon Dam in 1963,measurable flows from the Colorado River into the Sea of Cortez have occurred only infrequently.The wetland area at the mouth of the river has decreased from a historical average of 250,000 hectares to 5,800 to 63,000 hectares(depending on the year).In the Sea of Cortez,the lack of freshwater inflows has contributed to the endangerment of a large number of species.and the loss of algal productivity has caused the abundance of bivalve mollusk populations to drop 94 percent from 1950 values. gical Act of 19 Canyon Prote n Monitor rch Center to are Iof which are od o p05 experimental flood was generated to help scientists and managers i vestigate the effects of hiab flows on sedim rg ort and biolonical cultural and socioec Another set of experimental floods is nla d alon with aggressive efforts to reduce non-native trout onulations There is also discussion of installing a the nperatures below it.Partial restoration of historic ten tures below below the dam.More than 20 vears later.the number of species is as low or lower than before the restoration efforts began Further downstream,the number of insect taxa did increase,but only because warmer summer temperatures occurred in combination with periodic floods and sediment inputs from a tributary.8 Issues in Ecology Number 10 Winter 2003 BOX 2 — THE COLORADO RIVER The Colorado River is one of the most highly regulated and heavily used river systems in the world. Two principal reservoirs, Lakes Powell and Mead, along with 12 other large reservoirs store and release water according to complicated equations designed to maximize both hydroelectric generation and water supplies for agricultural, domestic, and industrial use in seven states across the Western United States and Mexico. More than 30 million people depend on Colorado River water. The original Colorado River Compact of 1928 allocated all water for societal use. (Actually it over-allocated because typical water volumes were overestimated while year-to-year variability was ignored.) Physical changes to the river below the dams have been profound. Flow in the Colorado River is snowmelt driven, and pre-dam flow patterns were dominated by large discharges from April through July, followed by low flows in late summer and fall. The river carried tremendous amounts of sediment from the highly erodible Colorado Plateau, and river temperatures were seasonally warm. Today, river flow is nearly decoupled from natural snowmelt, and peak discharges can occur in any month, often November to January. Daily changes in water releases as great as 566 cubic meters per second occur regularly for hydropower generation. Alluvial sediment, which once played a vital role in creating in￾channel habitat, is now trapped behind the dams, and the waters below are clear and sediment-starved. Also, because water is released from the bottom waters of most reservoirs, water temperatures for hundreds of kilometers below the dams are very cold throughout the summer and relatively warm during the winter, a reversal of the natural seasonal cycle. (An exception is Flaming Gorge Reservoir on the Green River in the upper Colorado basin, where water is released from multiple reservoir layers.) Ecological responses to the dams have been equally profound. The clear, cold tail waters below the dams, in conjunction with widespread introduction of non-native species, have promoted food webs that are alien to the Colorado River. Prior to regulation, the organic matter that fueled the river food web primarily originated on land and was carried into the river during large runoff events. Now, organic matter is supplied largely by luxuriant mats of algae that grow on the bottom of the river. The algae are consumed by insects and other invertebrates that historically occurred only in the much colder tributaries of the Colorado; these insects and invertebrates are in turn eaten by non-native rainbow and brown trout. Below the Glen Canyon Dam that holds Lake Powell, only four out of eight indigenous fish species remain, along with 22 non-native fishes, many of which either compete with or directly feed on the endangered native fish. Native cottonwood trees and the animal community they support are declining because the trees are unable to take root under variable flows. Also, upstream reservoirs that reduce the magnitude of annual floods prevent the establishment of cottonwoods higher on the riverbanks. Other shrubs and trees that are more tolerant of these modified conditions have grown profusely, including non-natives such as tamarisk. The effects of 14 major dams and hundreds of water diversions have been felt all the way to the river mouth. Since completion of the Glen Canyon Dam in 1963, measurable flows from the Colorado River into the Sea of Cortez have occurred only infrequently. The wetland area at the mouth of the river has decreased from a historical average of 250,000 hectares to 5,800 to 63,000 hectares (depending on the year). In the Sea of Cortez, the lack of freshwater inflows has contributed to the endangerment of a large number of species, and the loss of algal productivity has caused the abundance of bivalve mollusk populations to drop 94 percent from 1950 values. To reduce the impact of dam operations on the river’s ecological resources, Congress passed the Grand Canyon Protection Act of 1992. A large group of Colorado River stakeholders now work with a Department of Interior sponsored Grand Canyon Monitoring and Research Center to attempt through adaptive management to protect and restore riparian areas and native fishes, several of which are threatened or endangered. In 1996, after nearly 15 years of study, a large experimental flood was generated to help scientists and managers investigate the effects of high flows on sediment transport and biological, cultural, and socioeconomic resources. Another set of experimental floods is planned, along with aggressive efforts to reduce non-native trout populations. There is also discussion of installing a thermal control device on Glen Canyon Dam to raise water temperatures below it. Partial restoration of historic temperatures below Flaming Gorge Dam on the Green River, however, have not improved conditions for aquatic insects directly below the dam. More than 20 years later, the number of species is as low or lower than before the restoration efforts began. Further downstream, the number of insect taxa did increase, but only because warmer summer temperatures occurred in combination with periodic floods and sediment inputs from a tributary