WATERSHED — WHAT IS ONE?

The watershed of a river consists of all the land that sheds water into that river. It is separated from neighbouring watersheds by a rim of higher land. The entire watershed is tipped from the headwaters at the high end toward the low end at the mouth of the river. For example, the Salmon River watershed in eastern Ontario covers 921 square kilometers (227,579 acres). It is 380 metres above sea level at its high end and tipped down to about 75 metres at its low end at the mouth of the Salmon River at the Bay of Quinte.

 

In this example, the Salmon River watershed represents a large “drainage basin” within which the river and all its contributing creeks, wetlands and lakes drain. These smaller contributing areas may also be described as “sub-watersheds”, which are themselves defined by more localized boundary elevations. Similarly, the Salmon River watershed is a sub-watershed to the much larger Great Lakes watershed.

 

We all get water from two sources – the sky and the earth. Rain brings water that evaporated from oceans and from freshwater inland. Springs bring water up from the groundwater and we drill or dig to get groundwater from wells for our use. Once water is on the surface of the land, topography dictates where it flows.

 

Any topographic basin that catches water is called a watershed. The word implies that any water falling in that basin will be shed at the lowest point of that basin. Water can escape the clutches of gravity by evaporating but it can’t flow uphill. It can’t escape over the topographic edge of the basin so it must flow downhill until it can escape the basin at the mouth of the major river.

 

True, anywhere there is a leak down into the earth, such as a fault in the bedrock, some water escapes the basin and flows into the groundwater. Such recharging is the only way that our supply of groundwater can be resupplied to keep our wells filled.

 

All the other water that falls into the topographic basin flows downhill first in little rivulets, then in larger streams, then into the major river that drains that entire basin.

 

Chemical sources in the bedrock or the soils or man-made pollution entering the water of the upper watershed will flow downstream and affect the mid- and the lower watershed. This means that the headwaters and the upper watershed are the most important in controlling water quality of the entire watershed.

 

In addition to quality, the amount of flow in the mid- and lower watershed is strongly affected by the headwaters. If rainfall is heavy in the upper watershed, and there is little vegetation and no wetlands, a flood peak will flow down the watershed with forces sufficient to remove both natural (trees, boulders) and man-made (bridges, houses) structures. Vegetation in headwaters landscapes can blunt such flood peaks by both absorbing and by mechanically blocking the flow of sudden storms or rapid snowmelt.

 

Wetlands help to take the peak off any flood. Wetlands are small topographic basins that catch and hold rainfall or snowmelt. The water remaining after initial overflow is either held until it evaporates, is taken up by wetland plants or finds a way to recharge the groundwater. The result is to slow the speed of water flowing off the land. That removes the highest part of any flood peak and generally decreases severe effects of flooding in the entire watershed. These ecological, hydrological and geological processes are supplied by nature at no monetary cost.

 

For many, watersheds are the preferred land unit for conservation and management purposes. Watersheds are natural landscape units that don’t yield to the tangled bureaucracy of municipal jurisdictions. A watershed focuses action and policies on a land-based system of natural processes. Conservation Authorities in Ontario were based on individual watersheds for which basic soil, water, forest and wildlife information was gathered by crews of professional fieldmen and presented in a report for the watershed. If the people in the watershed were willing to follow recommendations in the report, a Conservation Authority was staffed and funded. The first one was the South Nation Conservation Authority formed in 1947.

 

Because all parts of the watershed downstream can be affected by the water flow from the headwaters, the area at the top of the watershed clearly is the most important part of the watershed system. In terms of natural processes, the headwaters area is the most valuable in the watershed. Commonly, this basic truth is not reflected in stewardship and conservation planning because the greatest number of people usually live further downstream, often close to the mouth of the major river. So political and monetary power overrides the basic importance of the headwaters area.

 

Although watersheds are integrated natural units, they are defined only by surface water flow. Movements of air, weather, animals, plants and people do not obey watershed boundaries. They cross them freely. So for other management purposes, the natural land unit may be defined by movement of wildlife, aerosol insecticides or Toronto residents crossing over the Oak Ridges Moraine to vacation beyond the watersheds that drain through the Greater Toronto Area.

 

However, for the majority of folks on the land, their watershed is mappable and understandable and encourages them to think about systems of natural processes starting simply with water flowing downhill.

 

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