Bull Run and its tributaries threated sewer infrastructure in Prince William County
Streams move their channels naturally within their floodplain. The energy of moving water erodes streambanks, changing the location where the stream curves. At times, a shift in the channel may isolate old paths and create "oxbows," named for their resemblance to the wooden frame used to harness oxen to pull wagons in the colonial era.
Patterns of riffles and pools move, as streams lower their base level are erode their bottoms. On the sides of streams, trees are naturally at risk of being undercut as channels migrate.
Minerals in bedrock near the surface gradually dissolve in water. Molecules go into solution and are carried downhill by groundwater and surface streams. Small particless of slow-to-dissove minerals, especially quartz (SiO2), are carried downhill by surface streams as particles of silt suspended in the moving water. Larger particles move slower as intact grains of sand, bouncing off the stream bottom in a process called "saltation."
As streams near sea level, the stream channels flatten and the speed of the water slows. Lower-energy streams can no longer carry all the suspended particles. They are deposited within the stream channel to form mudbanks and point bars on the sides, and to create a bed of sediment on the bottom.
At times of high water after storms, the volume of flow exceeds the capacity of the channel. That creates a flood, and water spills out onto the land adjacent to the stream. On those floodplains outside the stream channel, water spreads out and slows down, impeded in part by vegetation in the riparian zone. Slower water deposits much of the sediment it was carrying, creating new deposits on the floodplain. Sediment already on the floodplain can be moved around (remobilized) by stormwater, but except in massive storms the net result of a flood will be deposition of more sediment on the floodplain.
Erosion is a natural process, and the flow of sediment from mountains to the Atlantic Ocean has been going on ever since the supercontinent of Pangea split up about 250 million years ago. Natural erosion has created to topography of Virginia
Bull Run etches away at the streambanks between Prince William and Fairfax counties, shifting naturally within its floodplain
Agriculture and modern development have altered the speed and quantity of rainwater runing off the ground into streams. Faster runoff from impervious sufaces, especially rooftops and pavement in urban/suburban areas, results in unnatural flooding. Stream channels which had evolved based on a surrounding forest overflow after receiving more water in a shorter period of time. The extra water, with its extra speed, etches the sediments in a floodplain and causes streams to threaten nearby structures.
raindrops that land on parking lots race quickly to streams rather than soak into the soil
Stream restoration projects are deigned to reshape the channel to hadle extra flow, and to capture some of the runoff in stormwater ponds that slow the release of rainfall.