Hydromodification is the alteration of the natural flow of water through a landscape, and often takes the form of channel modification or channelization. A broader definition of hydromodification covers not just channel modification but also changes in land use or cover. Conversion of the open landscape to features such as roads, buildings, houses, sidewalks, parking lots, and flood control channels adds impervious surfaces and modifies runoff patterns, causing rainfall to run off into streams more quickly with higher energy, and large flow events to occur more frequently.
Hydromodification is one of the leading sources of impairment in streams, lakes, estuaries, aquifers, and other water bodies in the United States. Hydromodification activities such as channelization and channel modification, dams, and streambank and shoreline erosion control structures change a water body's physical structure as well as its natural function. These changes can cause problems such as changes in flow, increased sedimentation, higher water temperature, lower dissolved oxygen, degradation of aquatic habitat structure, loss of fish and other aquatic populations, and decreased water quality. It is important to properly manage hydromodification activities to reduce nonpoint source pollution in surface and ground water.
For a some real-world examples of hydromodification and its effects, see this Coastal Blog post.