A few impassable dams ...
Grand Coulee Dam; built 1941; 6,620mw; 550 feet tall; (with Chief Joseph Dam) Blocks 1100 river miles
Brownlee Dam; built 1959; 728mw; 420 feet tall; (with the two other Hell's Canyon dams) Blocks 600 river miles
These two groups of dams combine to block 55% of the Columbia River basin or about 90,000 square miles (almost the size of the United Kingdom)
A few impassable dams ...
Iron Gate Dam; Built 1962; 18mw; 164 feet tall
Copco No. 1 Dam; Built 1918; 20mw; 111 feet tall
Together with two other dams, they block over 300 river miles
A few impassable dams ...
Oroville Dam; built 1965; 645mw; 748 feet tall
A few impassable dams ...
Shasta Dam; built 1945; 676mw; 602 feet tall
Folsom Dam; built 1956; 207mw; 340 feet tall
Together these two dams block 187 river miles