Poor water quality in the city often occurs when the water is not deep enough. Especially in dry periods urban bodies of water can almost dry up completely. This can lead to fish mortality, which can be prevented by making the water deeper, so that there is always enough water for the fish and other water animals to survive.
To retain water during dry periods, parts of the waterbed can be made impermeable using boulder clay. In times of extreme drought and low water levels, water will remain in these areas, creating pools where fish and other water animals can survive.