The near extinction of beavers, the drastic reduction of wetlands, the frenzy of public works in the twentieth century, and other human actions have drastically altered hydrologic, geologic, and biologic systems.

Beavers build as many as 16 small dams per km of stream, creating wetlands; a single beaver dam can trap 6500 m³ of sediment, organics, and nutrients. Four hundred years ago there were between 60 and 400 million beaver in North America. By 1900, fur trappers had nearly eliminated the species from conterminous U.S.; today about 6 to 12 million beaver inhabit North America.

Approximately 894,000 km² of marshes, swamps, and bogs once covered 11% of conterminous U.S. About 53% of original inland and coastal wetlands have been lost to agriculture and development; they have been drained, filled, eroded, mined for peat, and inundated by reservoirs.

Humans have built large dams on almost every river; in the U.S. there are about 100,000 dams. The big dam era began in the 1930s; Grand Coulee Dam shut off one third of the Columbia River drainage basin to salmon. Dams change rivers to reservoirs, with accompanying changes in erosion and sedimentation, chemistry and temperature, fauna and flora.

Other examples of hydrologic alteration include an increase in impervious surfaces, dynamiting splash dams, channelization of streams, road construction, and dredging of the Intracoastal Waterway from Massachusetts to Texas. Human hydrologic changes have achieved goals but created problems. Fixes have included wetlands restoration, unchannelizing the Kissimmee River, and removal of the Edwards Dam on the Kennebec River.