Over here in the UK a weir is a sort of dam, sometimes fixed, but quite frequently adjustable in height which is used to control the flow or depth of a waterway. If you don't have them on a river, it will tend to flow fast and shallow and would not be very suitable for navigation by larger craft. On the upper Thames above Teddington (well there is a half-tide barrier at Richmond) there are weirs at regular intervals (usually about a mile or so) which keep the water deep enough for boats. Obviously these pose a barrier to navigation, so there is a lock at each to enable boats to get from one level to the other. This way the river is separated into a series of levels (or pounds) rather like stairs. The weirs are also used for flood control at times of heavy rainfall or flow.