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Mallard (Anas platyrhynchos), tufted duck (Aythya fuligula), and common pochard (Aythya ferina) still use the Brent reservoir in autumn and winter despite intensified sailing activities because part of the reservoir which is shallow and marshy is not accessible to boats. During the autumn of 1975 Canadian pondweed (Elodea canadensis) choked the northern arm and boating had to be suspended after August, resulting in a build-up of 159 common pochards, 30 tufted ducks, 17 northern shovelers (Anas clypeata), and 12 gadwalls (Anas strepera). Distances at which flocks of ducks would move from an oncoming sailing dinghy were estimated for tufted duck at 275 m and common pochard at 450 m, and small groups of smew (Mergellus albellus) to within 100 m. Mallard (Anas platyrhynchos) were similar to smew. Green-winged teal (Anas crecca), Eurasian wigeon (Anas penelope), and northern shoveler are more sensitive than mallard by an unknown amount. Green-winged teal and Eurasian wigeon have already stopped using the reservoir due to sailing activities. Perhaps screening vegetation on reservoir banks and provision of large floating vegetated rafts would reduce disturbance.