Weir - Drawbacks

Drawbacks

• Because a weir will typically increase the oxygen content of the water as it passes over the crest, a weir can have a detrimental effect on the local ecology of a river system. A weir will artificially reduce the upstream water velocity, which can lead to an increase in siltation.
• Weirs can also have an effect on local fauna. While a weir is easy for some fish to jump over, other species or certain life stages of the same species may be blocked by weirs due to relatively slow swim speeds or behavioral characteristics. Fish ladders provide a way for fish to get between the water levels.
• Even though the water around weirs can often appear relatively calm, they can be extremely dangerous places to boat, swim, or wade, as the circulation patterns on the downstream side—typically called "hydraulics"— can submerge a person indefinitely. This phenomenon is so well known to canoeists, kayakers, and others who spend time on rivers that they even have a rueful name for weirs: "drowning machines".
• The weir can become a point where garbage and other debris accumulates. However, a walkway over the weir is likely to be useful for the removal of floating debris trapped by the weir, or for working staunches and sluices on it as the rate of flow changes. This is also sometimes used as a convenient pedestrian crossing point for the river.