point be affected. Exactly the opposite effect will be experienced with the outgoing tide, but the change will still be from the outlet upstream. This picture is complicated by two factors. Firstly, the rivers entering the estuary are continually tending to exude water. They will be exerting a constant pressure on the water at their inlets. On the incoming tide they will tend to resist the backward flow of water, thus making the flow tide shorter (water can not flow upstream until this pressure is overcome) and to slow up the process of emptying the estuary, due to the large volume of water banked up upstream which is unable to flow away while the tide has been coming in. The second complication is that of the momentum of the moving body of water. The water in the estuary, moving downstream at low water, or upstream at high water, will tend to continue in that direction until it is stopped by back pressure. This will result in a slight piling up of water. This helps to account for the unevenness of the rate of flow, the piled up water surging in the opposite direction as soon as the pressure is released. (Fig. 2, 3.) Fig. 3.