Flooding is one of the largest environments hazard in Sri Lanka. It causes to the severe damage for human being and other structures. Lot of wetlands is situated in flat area in Sri Lanka. In the wetlands, water particle is come slowly to downstream because of the presence of the trees, infiltration and other losses. But in the reservoir water come quickly to downstream. So flood level of downstream is high and it occurs quickly. When wetlands are presence, there are many advantages except flood mitigation. It helps to degraded harmful materials. As example when some contaminators come to the wetland, it is degraded naturally with time because wetland has lot of factors cause for degradation. But there is reservoir, contaminators are not degraded naturally without harm to surrounding. So degradation of harmful contaminators is one problem we have to face. As well as increasing the temperature near the constructed reservoir is another problem forms with constructing reservoirs. Those some problems we have to face with constructing reservoirs.