Scottish freshwater lochs and reservoirs: a physical perspective

The extensive studies of the Bathymetrical Survey of the Scottish Freshwater Lochs (Murray & Pullar, 1910) provide a background for modern physical limnological research. This paper reviews such investigations in a range of lochs and reservoirs. Resurveying using electronic echo-sounders and radio position fixing largely confirms the original findings, establishing essentially very low rates of sedimentation. Geophysical sub-bottom profiling shows sequences of layered deposits upon irregular till surfaces. Sidescan sonar reveals suites of subaqueous landforms such as offshore terraces, depositional fans, scoured channels and blister-like slides in the organic-rich silts. The latter may be related to internal seiche movements of the summer thermocline. Shoreline terraces form in coarse marginal deposits in response to wind-generated, surface wave activity. Sidescan sonar also detects the presence of submerged walls, roads, buildings and former stream channels in artificial impoundments. Fully drawndown reservoirs permit direct visual assessment and excavation of bottom sediments deposited in known time periods. Grain-size changes in reservoir floor cores can be linked to documented agricultural changes in the catchments. Increases in ‘bare earth’ crop production are associated with coarser sediments. Soil erosion following pre-afforestation ploughing generates identifiable sediment increments on loch floors.