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The Enhancement of Drained Salmonid Rivers in Ireland 23 O.P.W. to provide guidelines for staff in relation to maintenance procedures for specific channel types which limit ecological change of value to fisheries and wildlife. 7. POSITIVE AND NEGATIVE IMPACTS OF DRAINAGE AND LESSONS LEARNED As one would expect the initial impact of arterial drainage on Irish salmonid streams was very serious with at least a temporary collapse of salmonid populations (Toner et a/., 1965, Ken- nedy, 1980, McCarthy, 1977 and 1983 and O'Grady unpublished). A review by the author of the only two sites which had been evaluated in detail pre- and immediately post-drainage many years later (21 an 30 years re- spectively) has illustrated a complete recovery in relation to their general ecology and, in particular, their capac- ity to support salmonid stocks (O'Grady, 1991(b) and O'Grady and King, 1992(b)). The findings in relation to the two zones in question probably reflect the situation in many channel sections of similar gradient (0.08%) where significant quantities of gravel and rubble eroded from the banks post-drainage recreating the pre- drainage channel bed morphology and where fencing, post-drainage, ac- commodated the re-establishment of a healthy riparian zone. Brookes (1992) has also noted a tendency for channels, particularly of median power (35 Wm 2), in Britain to recover post-drainage in a physical sense. Survey data compiled by the authors indicate that a failure of specific chan- nel sections to recover post-drainage in fisheries terms can generally be related to the absence of a riffle/- glide/pool sequence and/or any un- balanced riparian zone (excessive bankside vegetation or none at all). The physical nature of works pro- grammes which involved lowering river bed levels, thereby creating high banks, has meant that anglers must now fish while standing in the channel rather than standing on the banks. The author has had the opportunity of assessing the effects of draining whole catchments on atlantic salmon populations by comparing fluctuations in salmon numbers in two such drained watersheds with adjacent un- drained systems for a period of years pre-, during and post-drainage. Data indicate no relative change in salmon numbers in one case and a relative increase in salmon numbers post- drainage in the second drained watershed compared to the undrained adjacent watersheds (O'Grady, 1991(a) and O'Grady et al. 1993). Many individual factors played a part in preventing the collapse of the sal- mon populations in two of the afore- mentioned systems. They included the positive effect of removing a ser- ies of large weirs which had been ponding one system for 150 years, the removal of the natural bedrock protru- sions which also ponded channels, a significant increase in the length of gravelled channels exposed by drainage works, extensive post-