A review of fish response to spillways

Publication Date



fish response, juvenile, spillway, turbulence, injuries, gas bubble disease, flow deflectors, deflectors, plunging, air entrainment, entrainment, fish mortality, mortality, Canada, design

Report number

No. 1172

Publication place

Halifax, Nova Scotia


Department of Fisheries and Oceans

Series Title

Canadian Technical Report of Fisheries and Aquatic Sciences


Juveniles from most fish taxa will pass downstream, over or through spillways, if outlet flow is sufficient. Fish passing over relatively high spillways may be injured by rapid pressure changes, rapid deceleration, shearing forces, turbulence, abrasion and the striking force on the water in free fall. Injuries begin to occur when impact velocity of a fish striking a water surface exceeds 16 metres per second. Water below some spillways may become supersaturated with atmospheric gas to an extent that gas-bubble disease in fish may occur. Flow deflectors placed near the base of spillways have been successful in reducing the deep plunging action which is responsible for air entrainment and subsequent nitrogen supersaturation. Spillways are not currently perceived to be a serious source of fish mortality in Canada, although nitrogen supersaturation below spillways was identified as a potential fish-conservation problem in three rivers in British Columbia. Guidelines are presented which provide design and operating suggestions for spillways to eliminate or reduce their impact on fish.

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