What is a Watershed?



Watershed Definition:

“The term “watershed” describes an area of land in which waters drain downslope, along a drainage to a common lowest point or basin. Watersheds can vary in size, and every stream, tributary, or river has an associated watershed. Watershed boundaries can easily be delineated using a topographical map that shows the ridges associated with the various drainages and the mouth of the stream or river where water flows out of the watershed. The water drains via a network of surface and underground drainage pathways, and generally these pathways merge into a stream or river system that becomes progressively larger as the water moves downstream. Ground and surface waters may also merge or separate at various points along the pathway to the recipient water body that collects drainage from the watershed. Because the water naturally moves downstream in a watershed, any activity affecting the water quality, quantity, infiltration, or rate of drainage at one location in the watershed can change the physical, chemical, and biological characteristics at downstream locations. The physical connection between tributaries and larger streams and rivers, between surface and ground water, and between wetlands and all of the water sources, indicates the need to perform planning and management assessments at the watershed level.”


How are Watersheds Altered?

“Both natural events and human activities affect watersheds. Natural events such as storms, fires, and droughts can suddenly alter watershed conditions at large scales. While some natural events have negative impacts, these events are often critical for longterm ecological health. For example, a fire may damage a forest, but it also rejuvenates the forest by spreading seeds of key species and adding necessary nutrients to the forest floor. Individual human activities typically have smaller and more predictable impacts, but their cumulative impact can be far greater. Increases in population, land development, and economic activity increase demands for water, waste disposal, and raw materials. These activities increase pollutant releases to water and air and degrade or fragment natural habitats. Without appropriate management, these changes can seriously compromise watershed health.”


Human Activities that Impact Watershed Functionality:

Timber Harvesting

– Increase in impervious surfaces: roads, highways, paved pathways or driveways, roofs, result in rapid run off of precipitation causing water to leave the watershed faster with less infiltration

– Increases in run off can result in flooding, increased erosion and sedimentation, and loss of essential nutrients

– Increases in sedimentation negatively affects water clarity, drinking water resources, terrestrial and aquatic habitats and their associated organisms, and greatly impairs fish survival and spawning success

– Harvesting close to streams can alter stream bank stability, increase average water temperatures, and reduce nutrient cycling

Urban Development

– Increase in impervious surfaces

– Habitat loss (ex. installation of bridges and culverts on streams resulting in habitat loss)

– Flood management (ex. construction of dyke or armouring, streamlining water ways, causing increases in velocity and loss of habitat)

– Increase in pollutants, waste materials, and pathogens intercepted in watershed can cause negative impacts to native flora and fauna and human health