A weir is an impervious barrier constructed across a river to raise its water level on its upstream side and divert the water into the canal taking off from its upstream side. The top of the weir is kept below the H.F.L at weir is kept at F.S.L. [H.F.L high flood level – during foods water in the reservoir is allowed to the maximum water [MLL] it is called HFL]
If a were also stores for tiding over small periods of short supplied it is called a storage weir.
The main difference between a storage weir and a dam is only in height and duration for which main water is stored. A dam stores the supply of water for a comparatively long period.