How to limit river deposits to the river bed

Hi all
I'd like to know how I can limit the extent of river deposits (soft formation) to their river bed in a valley with steep slopes.
I tried using their mapped contours as his or polylines with and without tangent but the results are not what is to be expected: the base of river deposits extends into the valley slopes, resulting in the upper part of the slopes to be made of these river deposits...
Thank you for advising