The purpose of this paper is to discuss briefly the functioning of debris‐control structures in this region and the loss of storage‐capacity due to the accumulation of debris in local flood‐control reservoirs. In considering these subjects it is, I believe, essential that we have some conception of the why and the wherefore of the debris‐problem as faced here in Los Angeles County. In less arid localities there is little relationship between control of floods and control of debris. Here in Southern California, in some instances, when we control debris‐movement we go a long way in controllin