Over thousands of years, people have made changes to the physical structure of rivers, such as altering the course and installing flood defences and weirs. Many of these changes have benefited society, for example by improving navigation, reducing flood risk, generating power and powering mills, improving drainage or allowing water abstraction. However, there is also ecological damage resulting from the changes. Weirs block the migration of fish travelling back upstream to spawn. Straightened channels have low habitat diversity and cannot support diverse communities of invertebrates, fish and their predators. The biggest reason (29%) that waters in our catchments are not healthy is due to changes in their shape and flow caused by physical modifications.

Water transfers and reservoirs, such as the Gouthwaite reservoir on the River Nidd, have altered the natural flow of the rivers, interrupted sediment moving downstream and prevented fish and other animals from moving freely. While discharges and abstractions from reservoirs are carefully managed, river systems reflect the man-made influences on flows and sediment transport.

Where physical modifications are no longer necessary they should be removed and the river restored to its natural state to increase the quantity and quality of habitat. Weir removal will allow fish passage through the stream. An example of weir removal is the Reconnecting the River Laver Project on the River Laver, a tributary of the Ure that flows through Ripon.

Sometimes it is not possible to remove a structure because they are needed to provide a safe and reliable water supply or because the cost would be too high. It is sometimes possible to reduce the impact of these structures by installing fish passages or increasing instream habitat diversity.