Huge gains are expected in the river at Hartfield, the agency said
New gravel beds have been laid on a stretch of the River Medway in Kent to improve the habitat for wildlife including kingfishers and trout.
The work, on a three-quarter mile (1.2km) section of the River Medway at Hartfield, was done after monitoring revealed a decline in fish populations.
The Environment Agency said the work provided refuge and spawning sites for many native fish and invertebrates.
The gravel will increase water speed, allowing more oxygen into the river.
Insects use the gravel to live in and for feeding grounds and fish including trout use it to spawn.
It also provides refuge from predators to help the survival of young fish.
"The gains for the river Medway are potentially huge and we expect to see a wider range of fish species in the river," said Ben Lord, Environment Agency technical officer.
"This project will help ensure fish and wildlife in the Medway are able to sustain themselves."
Further improvements are planned for other stretches of the river later this year.