ABSTRACT
The landscape designer is sometimes required to control an existing watercourse or to provide a new artificial one, either as a feature on its own or as a supply for a lake or pond. The main problem with designing or modifying natural watercourses is the unreliability of supply; dry summers, exceptional rainfall, waterboards extracting groundwater, riparian owners extracting water for irrigation, and local development cutting off the aquifers supplying the watercourse are all hazards which affect the normal flow of water. In order to maintain water levels in the watercourse, shallow dams or small weirs should be placed at intervals across the stream, thus forming 'reaches' where the water level will remain at a safe level even when the flow is non-existent; a simple set of weirs is shown in Figure 8.1. If the watercourse is a natural or man-made stream serving other owners 'on-stream', dams may be built across such natural watercourses; these need permission and they must be provided with spillways, flood precautions and more careful control as required by the water authority. If the watercourse is a branch or 'leat' taken from a main stream, then 'off-stream' dams can be constructed without regard to other owners; and such dams usually have a controlled water supply which may be dictated by users of the main stream who will require a minimum flow to be maintained.
