 The changes in water flow and use caused by Hoover Dam 's construction and operation have had a large impact on the Colorado River Delta . The construction of the dam has been credited as causing the decline of this estuarine ecosystem . For six years after the construction of the dam , while Lake Mead filled , virtually no water reached the mouth of the river . The delta 's estuary , which once had a freshwater @-@ <unk> mixing zone stretching 40 miles ( 64 km ) south of the river 's mouth , was turned into an <unk> estuary where the level of salinity was higher close to the river 's mouth . 
