<p>As a result of analyzing theoretical, in situ, and laboratory research undertaken by famous scientists of the world, as well as our own studies of backwater siltation and sedimentation at low-head hydroelectric power plants (HPP) of lowland rivers, new patterns of these complex processes were established.We identified sedimentation patterns in a run-of-river reservoir based on the sediment balance and continuity equations. To construct a backwater curve along the upstream pool length, a simplified Chugaev – Rakhmanov method is recommended. The sedimentation process is considered taking into account three stages of channel transformation and the influence of sediment accumulation. Bedload was observed to deposit as a frontally advancing sediment wedge. Over time, the downstream pool bottom, as well as the bottom of the upstream pool rises in the immediate vicinity of the dam. The rise in the bottom elevation in the pools causes an increase in their water level as this leads to flooding of the dam water-discharge outlets from the lower pool side and, as a consequence, to a reduction in their discharge capacity. We developed a method for determining the discharge capacity of dam spillway structures as overflow spillways with a wide crest. Additionally, we proposed methods for predicting the siltation and sedimentation processes in backed-up pools of low-head HPP of lowland rivers based on the theory of river sediment balance and accumulation.</p>