 The dam is protected against over @-@ topping by two spillways . The spillway entrances are located behind each dam <unk> , running roughly parallel to the canyon walls . The spillway entrance arrangement forms a classic side @-@ flow weir with each spillway containing four 100 @-@ foot @-@ long ( 30 m ) and 16 @-@ foot @-@ wide ( 4 @.@ 9 m ) steel @-@ drum gates . Each gate weighs 5 @,@ 000 @,@ 000 pounds ( 2 @,@ 300 @,@ 000 kg ) and can be operated manually or automatically . Gates are raised and lowered depending on water levels in the reservoir and flood conditions . The gates are unable to entirely prevent water from entering the spillways but are able to maintain an extra 16 ft ( 4 @.@ 9 m ) of lake level . Water flowing over the spillways drops dramatically into 600 @-@ foot @-@ long ( 180 m ) , 50 @-@ foot @-@ wide ( 15 m ) spillway tunnels before connecting to the outer diversion tunnels , and <unk> the main river channel below the dam . This complex spillway entrance arrangement combined with the approximate 700 @-@ foot ( 210 m ) elevation drop from the top of the reservoir to the river below was a difficult engineering problem and posed numerous design challenges . Each spillway 's capacity of 200 @,@ 000 cu ft / s ( 5 @,@ 700 m3 / s ) was <unk> verified in post @-@ construction tests in 1941 . 
