In the summer of 1964 a study was carried out on the impounded Buttle and Lower Campbell Lakes located on Vancouver Island, British Columbia. An attempt was made to determine the effects of increased water level fluctuation on the limnology, particularly the bottom fauna, in areas with different substrate types. All data from two pre-impoundment studies were examined, and additional sampling carried out to provide a basis for comparison. In addition, several stations were chosen in each lake according to specific substrate types. These different bottom conditions were formed from the combined effects of erosion by water level fluctuation and wave action, over areas where different methods of pre-impoundment clearing of vegetation had been employed. The stations chosen were the shallow onshore areas which were exposed to the air during the winter months of minimum water level, the unexposed area immediately below the minimum drawdown level, and the pre-impoundment littoral zones in each study lake. A single pre-impoundment river channel station was located in Buttle Lake. In addition to bottom dredgings, funnel traps and nocturnal surface tows were used to sample the emerging insect fauna from each station. The total seasonal chironomid catch indicated significantly different total emergence between the several stations sampled. The pre-impoundment river station in Buttle Lake produced the highest total chironomid catch. Among the other stations the unexposed area immediately below the drawdown level was most productive. In Buttle Lake where the greatest seasonal fluctuation occurred and where the completely cleared littoral area was least protected from wave action, the total seasonal emergence over the exposed littoral area was very low. But in Lower Campbell Lake which received a much smaller seasonal water level fluctuation, total emergence was much higher over the exposed littoral. This area was cleared completely in this lake and the irregular shoreline afforded considerable protection from wind action. An inverted bathymetric distribution of the fauna was apparent with the greatest concentration occurring immediately below the drawdown limit in the unexposed littoral zone. The effects of water level changes were therefore considered the most important single factors influencing the bottom fauna distribution in each lake. / Science, Faculty of / Zoology, Department of / Graduate