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The Enhancement of Drained Salmonid Rivers in Ireland 19 More or less gravel may be ex- posed by works than one had antici- pated. For example in one Irish drainage scheme the works pro- gramme resulted in a 17% increase in the length of gravelled channel in a large catchment, a factor which was not anticipated (O'Grady, 1991(a)). The extent to which materials will erode into the channel is difficult to judge. Two years post-works the relative stability of banks should be evident, allowing one to make some assessment of what the longterm contribution of bank erosion to chan- nel morphology is likely to be in terms of sand, gravels and stone materials. Irrespective of the efficiency of ones silt entrapment procedures at works stage there is likely to be an unnatu- rally high silt load moving through the channel immediately post-works. Winter floods and routine mainte- nance over two years should scour out most such materials. Observations by the author indicate that in drained channels with gradient values be- tween 0.16% and 0.4% natural recovery of channel morphology is evident, though not complete, in shallow (< 1.0 m) gravel/stony bed sections two years after works with the reformation of a limited thalweg and a riffle/glide/pool sequence (O'Grady, unpublished data). At this stage one can see how such channel beds are evolving morphologically and design a physical enhancement scheme to accelerate this natural pro- cess and identify and correct imba- lances. 5b. Pilot Enhancement Pro- grammes In large catchments post-drainage en- hancement programmes may involve considerable expenditure. It is there- fore desirable that pilot experimental and control sites be established and monitored to ensure that particular ex- ercises will be productive in specific circumstances. In Irish rivers the rela- tive success or failure of such pro- gramme has been evident in most cases only one year after the comple- tion of the physical works stage even though optimum stocks of salmonids may not colonise such areas for 3 to 5 years after the works stage (O'Grady et al., 1991 and O'Grady et al. 1993). The importance of monitor- ing the effectiveness of enhancement programmes to assess their value in monetary terms is self evident. Also the usefulness of doing such so that one can develop new designs where original programmes failed should not be overlooked. 5c. In-stream and Bankside Phy- sical Enhancement Exercises The type of physical in-stream en- hancement exercises undertaken in Irish rivers and the relative success of some of these programmes have been outlined in a number of publica- tions (O'Grady, 1989, 1991(a), 1991(b), 1991(c), O'Grady et al., 1991 and O'Grady et al. (1993)). This in- vestigative programme is ongoing at the present time. The range of struc- tures and materials used and their de- sign functions are summarised here.