Estimates of the solid and solute loads passing 13 gauging stations on rivers draining the southern Grampian Highlands suggest solid yields ranging between 8.9 and 296.1 t/km2/yr according to locality. In the Tay basin the estimated sediment yield increases seawards, an anomaly partly attributed to organic matter in the water. Although biogenic suspended matter may originate in river waters, much is shown to be derived from micro-organisms in the lochs which abound in the basin. A direct relationship between the total surface area of the lochs and the sediment yield of the catchment is suggested. Although 1.72 × 106t and 1.9 × 105t of solids enter the Tay and Forth estuaries annually, more than 50% of that may be of organic origin.