Summary:

Despite the various categories of defining the life of water reservoirs, the notion life of reservoir most often relates the period of the operation to such silting degree that makes impossible the realization of the tasks of the reservoir. In this work it was show the change of the value of the silting degree and sediment trap efficiency of chosen reservoirs and was estimates the time after which small water reservoirs will be silted in 50%. This is the period, which by Pitt and Thomson [1984] is defined as HLT, i.e. half-life time. The following reservoirs were chosen to studies: Krempna on the Wisłoka river, Zesławice on the Dłubnia river and Cierpisz on the Tuszymka river. The elaborated silting forecast of the change of the silting degree, make by the Gončarov's method [Wiśniewski, Kutrowski 1973], made possible the qualifi-cation the HLT silting time of small water reservoirs. It was stated that the life of studied reservoirs carried out from 17 to 40 years. The differentiation of the HLT value results from the hydraulic conditions of the inflow and flow through the re-servoir of the water and sediment which one can characterize in the very general method using the sediment trap efficiency (β). It was stated, that reservoir with the small reduction on β value is characterized by the longer life.