Chalk streams are the richest of all the types of river and can provide superb fishing. These rivers are fed by rainwater slowly draining through chalk rock and then rising to the surface again through springs. Most chalk rivers are carefully protected trout fisheries and offer some of the finest dry fly-fishing in the world. The water is usually swift flowing and very clear. The temperature of these rivers does not fall very low even in winter and they are rarely subject to serious flooding. The luxuriant weed growth harbours the nymph stage of many species of fly. Coarse fish do exist in chalk streams and can grow to specimen proportions. Because chalk streams are normally managed as fly-only trout fisheries, coarse fish are treated as a nuisance and netted out to reduce competition with the trout.