Gullies are a part of a drainage basin.
A drainage basin is a topographical area consisting of a river and all its tributaries. Such tributaries include not only streams but also groundwater, runoff and other channels of flowing water. Smaller drainage basins may be contained within larger ones. For instance, the Mississippi River and its tributaries constitute an enormous drainage basin; the Missouri River, while part of the Mississippi system, is also a drainage basin in and of itself. In North America, drainage basins may also be referred to as watersheds.
Divides
Hiking trail straddles the Continental Divide, the most well-known divide in the U.S.
Drainage basins are separated from each other by divides, which are areas of high elevation, often hills or mountains. Precipitation falling on one side of a divide will flow into a different drainage basin than precipitation falling on the other side of a divide. The most well-known divide in the U.S. is the Continental Divide. This invisible line runs through the peaks of the Rocky Mountains, separating streams and rivers flowing west into the Pacific from those flowing east into the Gulf of Mexico.
Groundwater
Rain seeps below the Earth's surface to become groundwater.
Originating as rain, snow or ice, groundwater consists of precipitation that has seeped below the Earth's surface. It fills aquifers, which are naturally occurring porous or cavernous underground reservoirs. Like the streams above it, groundwater also flows into larger rivers, lakes or the sea, contributing an additional volume of water to the composition of a drainage basin.
Overland Flow and Runoff
Rainfall is initially absorbed by the soil, but when the ground becomes saturated, gravity pushes the accumulated water across the surface in flat sheets. This is known as overland flow. Eventually the overland flow reaches a larger collection of water such as a stream. Oftentimes the terms overland flow and runoff are used interchangeably. Scientifically speaking, however, overland flow is a type of runoff; runoff also consists of groundwater and a few other minor sources.
Rills and Gullies
Overland flow eventually organizes itself into small channels called rills. Rills form more quickly in areas that lack vegetation and they disappear shortly after rainfall has ceased. Rills eventually converge to form gullies, which are ditches that funnel water into streams. From here, water continues its downhill flow until reaching larger creeks and streams.
Floodplains
Floodplains can occur along winding rivers.
Floodplains may occur along large, winding rivers, often near their outlets to the sea. These flat areas may be inundated with water during the springtime or after prolonged heavy rains in the drainage basin. In some areas of the world, flooding occurs at the same time each year. The soil in floodplains is very fertile and conducive to crop-growing, thanks to sediments deposited from further upstream.
Tags: drainage basin, below Earth, below Earth surface, Continental Divide, Earth surface, Floodplains occur, Floodplains occur along
