The shape of a river channel reflects the pattern of the waterflows than run down through that channel, as well as the rock through which the water has carved its path. The width and depth of stream channels is determined by the amount of water carried by the stream after a 2-year flood (the rainstorms with a 50% chance of occuring). Floods occur when the volume of the river channel is not sufficient to contain the runoff from bigger storms.
A river will carve a channel with clearly-defined banks on each side, in most cases. During occasional floods, the water level will rise to fill the channel. Newspapers may describe the river as running "full to the banks," but the river does not reach flood stage until it rises higher than the river banks and water spills over onto the adjacent land.
When the river level starts to drop, perhaps after the surge of water from an intense thunderstorm has passed, the river is described as having "crested."
On August 30, 2004 in Richmond, when Tropical Storm Gaston dropped 12 inches of rain in one afternoon onto downtown Richmond. Shockoe Creek filled up and water overflowed the stream banks, covering the Shockoe Slip restaurant district up to the second story of some buildings.
Weather forecasters were surprised that the routine tropical storm traveling north from the Caribbean would stop and create such heavy rains in a small area. Normally, such storms dump just an inch or three on any one place before moving through the area, but Gaston kept on dropping water in the same local area.
Virginia residents are accustomed to about 44 inches of rain per year, and the vegetation reflects it. It takes only a few years for grass, shrubs, vines, and/or trees to create a green cover on Virginia's soils, thanks in large part to the rainfall nurturing new plants. The Atlantic Ocean and Chesapeake Bay moderate the Virginia climate, too. In contrast, the dry California desert still shows the scars of tank tracks from military training exercises during World War II, while the human-caused surface disturbances in Virginia are obscured within months in many cases.
Floods are becoming more common, independent of any change in the climate due to global warming, because our developments are modifying patterns of runoff and the shape of the river channels. When a river channel is narrowed, often in an urban area by filling in the marshes on the shoreline, the volume of the channel is decreased. The inevitable consequence, according to the laws of physics, is that the water must either flow faster (eroding the riverbanks and causing trees/structures to fall into the river) or rise higher (causing floods when waters spill over the riverbanks).
Even if a channel is unchanged by storm-induced erosion, development can increase the probability of floods downstream. When forests and grasslands are replaced by impermeable surfaces, such as parking lots, roads, and buildings, the rainfall no longer seeps gradually into the ground. Instead, it rapidly flows to the edge of the pavement or roof. There it may have sufficient energy to wash away soil and vegetation and carve gullies, washing silt into streams and causing water pollution.
More importantly for flood control, the rapid flow of stormwater off the surface of the land creates a higher peak flow downstream. For example, look how a one-inch summer thunderstorm in a small stream valley would create a small rise in water level at a stream gauge downsteam. The storm would deposit an excess of water that could not be absorbed in the ground. In this example, assume that the excess normally would flow downstream over the course of 24 hours, and cause the water level at the stream gauge to rise to a peak of 6" above average flow four hours later.
Building a shopping center in that small stream valley would not change the total amount of water deposited by the same storm. However, covering the ground with impermeable surface would dramatically speed up the the time during which the excess rainfall washed downstream. Instead of taking 24 hours to drain, the excess water may race off the roofs and parking lots into gutters, and be gone soon after the rain stopped (except for a few puddles). The runoff would be faster and the peak water level at the stream gauge would be higher.
in 2010, people ignored "Down Drown, Turn Around," and the swift water rescue team went to work in Fairfax County
Source: Federal Emergency Management Agemcy (FEMA), My Time in the Water: Flood Safety Lessons Learned
A "flashy" stream has an unusually rapid rise in water level and then a quick drop back to the height of the average flow. Over time, such streams will carve deeper channels, incising themselves into the floodplain and creating riverbanks that erode during every storm. Trees and paths along such riverbanks are vulnerable to erosion - look for an unusual number of trees tilting over streams in urban parks, and you're probably seeing signs of runoff that was accelerated by upstream development.
The normal water cycle in Virginia is not always normal. Water in Virginia can cause its own dramatic changes in the surface. The effects of Hurricane Agnes in 1972 are still visible in Nelson County, and the Madison County floods in 1995 substantially reshaped the stream channels in that part of Shenandoah National Park.
In many areas of the United States, the spring thaw brings on an annual flood as the winter snows melt. When the water flows over the riverbanks, it encounters obstructions - grass, trees, fences, even barns and houses. The obstructions slow the flow of the water, and some of the silt carried in the floodwaters settles out along the edge of the river. The deposited silt fills in the low places near the river edge. In the resulting flat floodplains, the soil is rich in organic material. Floodplains are desirable farmland - the soil is fertile, enriched by nutrients eroded from upstream, while seeds/plants on the flat land are less likely to be washed away by normal rainstorms.