This is a cartoon of a watershed (aka drainage basin, catchment). The blue lines are streams or rivers. Streams may only carry water during rainstorms and be dry at other times. The dashed line represents the watershed boundary. Rain or snow that falls within the watershed boundary can flow through to the river network and eventually reach the river mouth – at the bottom of the figure – because of differences in elevation. Rain or snow that falls outside the watershed boundary will drain a different watershed. Figure: Zimbres 2005.

The area of land where all the precipitation (rain and snow) drains to a common outlet is called a watershed. The term watershed is also used interchangeably with drainage basins or catchments; for the rest of this post, I will use watershed.

Watersheds are defined by the characteristics of the landscape. Water flows downhill, so features like ridges, hills, or mountains separate different watersheds.

For my work, a watershed is a “unit” of study. Large watersheds contain many smaller watersheds. How you define a watershed boundary depends on your interest. For example, when I studied nitrogen fluxes to the Gulf of Mexico, the relevant "unit" was the entire watershed of the Mississippi River (the green-colored area in the figure below). This watershed is huge and occupies 40% of the area of the continental US. If I had only wanted to want to look at just the Ohio River, the area would have been much smaller.

The Mississippi River drains about 40% of the area of the continental US (green shaded areas). The watershed can be divided into a number of smaller watersheds (gray lines). For example, this figure shows seven sub-watersheds, but there are other ways of defining them.

Water connects

Toxic algae bloom in Lake Erie, an aerial view. Photo: Jesse Allen and Robert Simmon - NASA Earth Observatory.

It is probably an occupational hazard, but I can't help but think about how water moves across the landscape, from soils to lakes and streams, perhaps reaching the coastal ocean. This has changed how I look out the window when I travel, especially by airplane. From a bird’s eye perspective, you can see the patchwork of cropland, cities, suburbs, forests, and lakes and the waters that connect them.

Water is a critical resource that can't live without

Competition for important resources, including water, can result in conflicts and disputes. When an upstream party uses water for drinking or irrigation or builds a dam, there could be less water in a river for downstream parties. An example of this type of dispute is currently going on between Florida and Georgia, over water rights to the Apalachicola River.

Another type of dispute relates to upstream pollution that degrades downstream water quality. For example, in Des Moines, Iowa, the nitrate concentration in drinking water exceeds legal limits because of contamination with fertilizer runoff from cropland. This situation requires the water utility to spend extra money to treat the water. As a result, the utility filed a lawsuit against three upstream counties.

Watershed governance

To jointly manage shared water resources, there are many agreements and governmental-type organizations that bring states, countries, or different parties together.  Examples include:

• Seven western US states agreed to the Colorado Compact in 1922 to allocate the water of the Colorado River among the states.
• In the Baltic Sea region, the Helsinki Commission (HELCOM) governs the Convention on the Protection of the Marine Environment of the Baltic Sea. The nine countries that border the sea plus the EU are parties to the Convention.
• For the Great Lakes and St Lawrence River, there is the Great Lakes Commission. Eight US states and two Canadian provinces are members of the Commission. They work "...to develop shared solutions and collectively advance an agenda to protect and enhance the region’s economic prosperity and environmental health.”

On the local scale, small watersheds can be a focal point around which citizen groups organize. Watershed councils build relationships within communities, provide environmental education, and coordinate volunteer restoration efforts, among other things. If you are interested in water issues, watershed councils are an excellent place to get involved.

In the US, you can find your watershed here or here.

Text: Michelle McCrackin