In this paper we calculate the trap efficiency of the reservoirs in the Siret Basin and we make also a briefly analysis to showthe way this reservoirs influence the sediment discharge of the rivers by sedimentation. Trap efficiency (TE) is an indicator and parameter of sediment yield from upstream and of sedimentation rate from the reservoirs. In consequence we calculate for 14 reservoirs in the Siret Basin this parameter. The values of TE vary between 42 – 98% and correlates very well with the capacity of the reservoirs. In reservoirs with the high capacity the values approach to 100% while in small reservoirs, the value of TE is decreasing. Capacity and operating mode lakes are important factors controlling the degree of retention of sediment fromthe source area. Sedimentation within reservoirs is a problem as it decrease the storage capacity and, hence, makes the structure less efficient. For small reservoirs, sedimentation can become a severe problem as the rate of siltation is generally much higher in comparison to large dams.