One simple way in which to enliven a stretch of river, making it more suitable for invertebrate life and fish habitat, is to introduce individual sections of gravel riffle along its length. These shallow, fast flowing sections naturally entrain oxygen into the water, as well as providing good spawning habitat for salmonids.
They are relatively cheap to install, but need to be carefully sited and engineered to ensure banks alongside do not wash out. Gravel size will be dictated by the river characteristics and the species of fish to be encouraged. Often a gravel riffle can go hand in hand with a two stage channel, concentrating flows during the summer months but allowing the river to expand its width during spate conditions without overtopping.
It is surprising how quickly this simple procedure, perhaps with the addition of some low cost timber deflectors to influence flow patterns, can generate marked improvements to the ecology, (plants, invertebrates and fish) of a river.