The three main fishway types, pool-and-weir, vertical slot, and Denil fishways, are formed by a sloping channel equipped with weirs placed at a certain distance from each other. Weirs of the various fishway types differ from each other, giving the fishways their own character. It can be said that flow over (or through) weirs characterizes fishway flows. There is, however, one remarkable difference between weir flows and fishway flows: Flow over weirs deals with a single weir only, with the only factors affecting the flow, in addition to weir characteristics, being the approaching velocity and submergence. In fishways flows, the flows over the adjacent weirs may affect each other. Despite that, flows over weirs can be used to some extent in analyzing fishway flows.
Of the many hydraulic structures developed by man, the weir is perhaps among the oldest. From history it appears, that weirs were first constructed in India, Ceylon, Egypt, and China, and some of them date back to the period before Christ (Lakshmana Rao 1975). Weir flows have been studied intensively and more extensively covering their role in flow measurement and control, velocity control, free overfall and submerged flow, and several weir structures (e.g. Rouse 1936, Rand 1965, Vanden-Broeck & Keller 1987, Dias et al. 1988, Dias & Tuck 1991, Rubin 1997). In addition, hydraulic jumps and jet flows as a part of weir flows have been under consideration in several studies (e.g. Rajaratnam 1965, Ohtsu et al. 1990, Long et al. 1990, Wu & Rajaratnam 1995).
Fishways can also be considered to be roughened open channels, where flow resistance plays an important role. Calculation of the flow resistance is one of the main problems in hydraulics. For example, Rouse (1965) has given a thorough and comprehensive description of the mechanism of flow resistance. In Finland, for example, Hosia (1978) has studied the resistance of nonuniform flows. Equations for computing and evaluating the rate of flow usually contain many uncertainties, and it has been stated that there are arguments for using almost any flow resistance equations (Veijalainen 1985).