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18 M.-F. O'Grady and J. Curtin nels after works. In more recent years O.P.W. have also reseeded dredged banks with grasses and replanted trees in addition to fencing. When drainage schemes were completed maintenance of fences became the responsability of individual land owners some of whom maintained these structures while others did not. A review of the riparian zones by the authors along rivers drained in the 1950's and 1960's indi- cates that where fences were main- tained post-works a healthy ecological riparian zone with a continuous treeline and associated marginal flora has re- established itself without any replanting programme even where both banks had been stripped of all vegetation at works stage. The nature of these tree- lines varies - in some cases a virtual monoculture of Alder or Willow species has regenerated, while in other circum- stances mixed stands of species evi- dent in the surrounding landscape are present. On properties where riverside fences have not been maintained post- drainage and lands have been used for grazing a healthy riparian zone has not regenerated post-drainage often result- ing in bankside erosion and bank slip- page where livestock have ready access. These observations clearly illustrate that the fencing of channels should be an integral part of drainage operations at works stage. The value of maintaining an undisturbed riparian zone, in both fisheries and general wildlife terms, even in undrained channels has been highlighted by many authors (Waters, 1968, Hynes, 1970, Karr and Schlosser, 1978 and others). 4d. Retention of Excavated Bed Materials Significant quantities of stone materi- als may be required to carry out the post-drainage fisheries rehabilitation programme. It is suggested that the material from bedrock areas and spawning gravels excavated in the course of drainage works be retained for this purpose where possible to re- duce costs. 5. POST-DRAINAGE FISHERIES REHABILITATION PROGRAMMES 5a. A Strategy Apart from the bank fencing, reseed- ing and planting programme it is rec- ommended that physical in-stream fishery rehabilitation works be delayed for two years post-drainage for the fol- lowing reasons. While backwater analysis allows one to predict the likely effects of the drainage pro- gramme, the recovery of the channel is dependant on a number of ecologi- cal, physical and hydrological factors which may interact in unforseen ways post-works. Modifications to drainage design may be required at works stage to ac- commodate other public or private in- terests e.g. abstractions or local amenity parks. Consequently the exact nature of the works schedule and its effects on the channel in physi- cal terms cannot always be antici- pated.