GEOMORPHOLOGICAL APPROACH TO THE RIVER RUNOFF EVALUATION IN THE GEOLOGICAL PAST (Paper 1. Regime Equations)

Reconstruction of river palaeodischarges based on the morphology of modern rivers meets the following requirements to the empirical regime equations in use: 1) they should cover the broad range of environment conditions to include the ancient conditions of river formation; 2) they should be controlled by a small number of variables selected according to the needs of the problem; 3) the option must exist to use a relationship that is suitable for the palaeoenvironmental conditions. These principles were applied to calculate discharges that formed the Lateglacial palaeochannels 5–15 times wider than the present-day rivers. The calculated mean annual palaeodischarges were only 2–4 times larger than those of the modern rivers. These palaeodischarges have been produced by an annual rainfall of about equal to or only slightly larger than the recent one. The main environmental conditions were: 1) a long winter, the accumulation of sufficient (300–700 mm) amount of water equivalent in snow; 2) short and sharp flood; 3) small water losses during this flood due to low soil permeability under permafrost conditions; 4) long low-water period, when these large channels were almost dry. Therefore, the mean annual discharge appeared to have risen substantially less than did the flood discharge.