13.9. FISH LADDER Various kinds of fish are present in large rivers. Most of these fish migrate from the upstream to downstream in the beginning of the winter in search of warmth and return upstream before the monsoon for sediment-free water. While constructing a weir across a river, steps have to be taken to allow for the migration of fish. For this purpose, a narrow opening between the divide wall and the undersluices (where water is always present) is provided. Most fish can travel upstream, if the velocity of water does not exceed about 3.0 m/s. Hence, baffles or staggering devices are provided in the narrow opening adjacent to the divide wall. These openings are called fish ladder (or fishways or fish pass), (Fig. 13.8). It is advisable to know the requirements of the fish of the river, and the design of fish ladder should take into account these requirements.
