Abstract: Formation and evolution of shallow lake deltas are an important alluvial process of a river system, and research on the waterway and morphology over such deltas has become a key issue of river dynamics. In this study, a tank experiment based on the natural model method is conducted to simulate the formation and evolution of a shallow lake delta, focusing on the influence of boundary conditions, i.e. different sediment inflows and different lake stages, on its plane shape and stretching in longitudinal, vertical and horizontal directions. Results reveal that the delta first stretches forward like a tongue and then evolve as a developing lobe. Its aspect ratio of length to width increases rapidly first but approaches a relatively stable stage; the more the sediment inflow, the smaller the aspect ratio; a higher lake stage suppresses more the size of the delta. Both its longitudinal stretching and lateral broadening manifest a rapid growth at the initial stage, and then it enters a relatively stable mode of stepped growth. Higher sediment concentration leads to faster advancing longitudinally and laterally, resulting in an earlier start of the stepped growth mode; a higher lake stage leads to a similar earlier start with a smaller delta size, a longer mutation period, and a smaller delta size increment in one step. Sediment often silts over the lower delta. However, more incoming sediment will cause silting over the upper part. This brings about a decrease in the upper channel depth, an increase in the lateral bed slope of the middle and lower reaches, and an increase in the delta’s overall longitudinal slope.

Key words: lake delta, natural model method, plane shape, longitudinal advance, transverse broadening, water level