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16 M.-F. O'Grady and J. Curtin species such as Nuphar and Potamogeton natans sp. which have been observed growing in deeper water (<_ 4 m) Caffrey (1990). If, as is likely, a drainage scheme will involve the lowering of river bed high points it is important that biological data compiled in the pre-drainage survey, in relation to the specific maximum depth of the euphotic zone, be taken into account in adjusting the first phase design. The authors have ob- served a number of situations in drained Irish rivers where, with hindsight, if bed high points had been lowered by an additional 0.5 m exten- sive length of channel upstream of these impoundments would have developed a significant flora post- works and became more valuable in a fisheries sense (O'Grady, 1989 and 1991 and O'Grady et al. 1991(a) and O'Grady et al. 1993). This point has now been accepted by Irish drainage engineers in relation to designing fu- ture schemes (Howard, 1993). In a major study of the effects of channelization on bank stabilisation in 46 partially drained rivers in Britain Brookes (1987) has shown that bank erosion is one of the most common problems in the lower undrained reaches of such systems. Pre-drai- nage physical data in combination with predicted hydrological change post-works should allow one to anti- cipate such changes and plan for ad- justments as required. Where drainage objectives can be achieved by excavating from one bank only then the disturbance should take place on the north facing bank where possible in order to maintain a channel shade factor. This is particu- larly important in specific circum- stances where channel bed type is particularly suited to colonisation by dense stands of Ranunculus or Pota- mogeton sp. (fine gravel/sand matrix). Care should be taken to dredge the channel from the north facing banks only where possible to maintain a de- gree of shade and avoid profuse growths of such plants in the early post-drainage phase. Where channel sections of this type are flowing ge- nerally east/west in direction and the removal of the riparian zone on one bank is unavoidable then replanting of the dredged bank should be given an absolute priority. The authors have ob- served sections of drained channels in Ireland were a treeline on the south facing bank did not regenerate post- drainage resulting in a weed chocked channel in summer flow conditions. Electro-fishing data indicate that, such channels cannot support significant salmonid populations in summertime (O'Grady and Caffrey, 1984). 4. ACCOMMODATING FISHERIES INTERESTS AT WORKS STAGE In studying the effects of drainage on Irish rivers the authors, based on their experience, would make a number of recommendations in this area.