Summary of Hypolimnetic Withdrawal
Reservoir nutrient cycling and anoxic conditions
(and associated concerns) in the hypolimnion.
Mode of action
Removal of hypolimnetic waters through bottom
Effective on small lakes with strong stratification,
dependent on lake volume.
Removal of hypolimnetic zone will reduce or
eliminate habitat for cold-water fisheries.
Downstream release water will typically be of
Applicability to reservoirs
Applicable to reservoirs with withdrawal facilities
whose intakes are located in the hypolimnetic
region of the reservoir.
4.3.3 UNDERWATER SKIMMING WEIR OR SUBMERGED CURTAINS
220.127.116.11 Problem Addressed
Reservoirs that exhibit strong thermal stratification with a cool hypolimnion throughout the
summer months may be able to maintain a cool-water fishery in the hypolimnion or in the river
downstream. If the project operates a bottom withdrawal structure, such as a hydropower project, the
cool hypolimnetic water can be deplete during the summer months. Therefore, maintenance of a cool-
water fishery is difficult, if not impossible. A temperature control curtain or submerged skimming weir
(underwater dam) can retain the cool water by preventing or limiting its withdrawal and release
downstream (Figure 4.3.3). If a curtain is used, it can be lowered to allowing withdrawal of "stored"
cool water in the late summer, thereby maintaining water quality for a cool water fishery. However,
underwater dams may hinder reservoir drawdown, act as a sediment trap, and impede fish migration.