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.