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The Enhancement of Drained Salmonid Rivers in Ireland 15 in its banks. A review of the first phase design will determine whether or not the proposed excavation level is likely to alter the substrate type thereby im- pacting on the ecological regime. Data on bank materials linked with gradient values will indicate the extent to which erosion might take place post-works and the type of material likely to be washed into the channel. Obviously the erosion of some stony bank materials post-works would be advantageous while a significant in- crease in sand/silt scour would be dis- advantageous. This data base will enable one to assess the relative ex- tent to which bank protection works might be required in specific channel areas. If the engineering studies indicate that drainage design will necessitate a significant broadening of channel base width the it is imperative that final design facilitate the construction of a "channel within a channel" so that pre-drainage base width at summer levels can be retained within the full drainage base width in order to avoid braiding. Fishery survey data will also allow one to define the location and extent of salmon and trout angling zones where considerable excavation is planned and provision for angler access post-drainage will be required. Alterations of bankslope design would be considered here with side slopes of 1.5 : 1 being planned. More gently sloping banks providing increased dis- charge area will also allow for a re- duced design base width. Petersen et al. (1992) has shown that reduced side slopes may also reduce longterm maintenance costs. Drainage will sometimes involve a lowering of bed rock protrusions. In the authors experience the design of the cut through the rock shelf and the nature of the bed post-works will sig- nificantly influence its capacity to sup- port salmonids (O'Grady et al., in press). A stepped bed in such circum- stances will provide pool areas for trout or provide pockets which can, post-works, be filled with gravels for spawning purposes. In less severe gradients (< 0.3%) a smooth rock bed covered with a layer of large stones will significantly increase the salmonid carrying capacity and greatly increase the standing crop of macro-inverte- brates (Needham, 1969). The place- ment of rubble on a smooth rock bed may require adjustment of the design bed level and/or base width to accom- modate the increased roughness coeff icient. The authors have noted a variation in the light penetration level in Irish rivers which is dictated by water clarity and related to individual catchment characteristics. A significant variation in the euphotic zone in Irish rivers has been noted as a consequence of this phenomenon with aquatic plants being limited to channel bed areas re- ceiving adequate incident light at mean summer low flows. The range of depths at which plants (algae, mosses and macrophytes) can grow in Irish rivers, as observed by the authors, varies from 0.23 m to 2 m. This generalisation excludes the spe- cialised floating leaved macrophyte