Understanding Watersheds: Key Elements and River Basin Types
Understanding Watersheds
A watershed encompasses several key elements:
Watershed Divide
The watershed divide is a line that delineates the watershed, marking the boundary between it and adjacent watersheds. Precipitation on either side of the divide flows into different rivers. It’s also known as the divortium aquarum.
The Main River
The main river is typically defined as the watercourse with the greatest volume of water (average or maximum), the longest length, or the largest drainage area. Defining the main river and its source can be arbitrary, as is distinguishing between the main river and a tributary. Most drainage basins, however, have a well-defined main river from the mouth to near the watershed.
The main river course is the distance between the source and the mouth. A river’s course can be divided into three parts:
The Upper Course
Located in the highest relief, the upper course is where vertical erosion dominates, resulting in the deepening of the channel.
The Middle Course
In the middle course, the river begins to meander, widening the valley.
The Lower Course
The lower course is located in the lower parts of the basin. Here, the river’s flow loses strength, and solid materials settle, forming floodplains or valleys.
Other Important River Terms
- Cauce (Channel or Bed): (From Latin calix, icis, feed tube.) The riverbed or stream; the channel through which water flows for irrigation or other uses.
- Thalweg: The line connecting the deepest points along a watercourse.
- Right Bank: Looking downstream, the margin on the right.
- Left Bank: Looking downstream, the margin on the left.
- Downstream: Relative to a section of a watercourse, whether main or tributary, downstream refers to a point after the section under consideration, moving in the direction of the current.
- Upstream: The opposite of downstream.
Tributaries
Tributaries are secondary rivers that flow into the main river. Each has its own tributary basin, called a sub-basin.
Water-Parting
The water-parting is the line between two or more neighboring basins, often used as a boundary between geographical areas or watersheds.
Relief of the Basin
The relief of a basin consists of the main and side valleys, with the landforms and the major and minor river systems that form a watershed. It includes the mountain and its flanks, streams or rivers, and valleys.
Human Impact
Human works, also called anthropogenic interventions, observed in the basin typically include homes, towns, farmland, irrigation works, energy infrastructure, and roads. Human activity is often the cause of disasters in the basin due to overexploitation of resources, deforestation, and construction in floodplains.
Parts of a River Basin
Upstream
The upstream part of the basin is where erosion predominates. There is a contribution of earthy material towards the lowest parts of the basin, and visible traces of erosion are present.
Middle Basin
In the middle basin, there is a balance between the solid material entering and exiting the stream. Erosion is not readily apparent.
Lower Basin
The lower basin is where material removed from the upper part is deposited, forming what is called an alluvial fan.
Types of River Basins
There are three main types of river basins:
- Exoreic: Draining its waters into the sea or ocean (e.g., the Rímac River).
- Endorheic: Flowing into lakes, lagoons, and salt flats that have no outlet river to the sea.
- Arreicas: Waters evaporate or seep into the ground before being channeled into a sewer. Streams, washes, and canyons of the central Patagonian plateau are of this type, as they do not drain into any river or other hydrographic body of importance.