Watersheds

Watersheds

Watersheds differ in size from tens of acres to a million acres Watersheds differ in size from tens of acres to a million acres. It all depends on the structure of the landscape. The black lines here are watershed divides surrounding four separate watersheds.

Watersheds are given geographic names (just like counties.)

What is a Watershed? (which is sometimes called a drainage basin) 1. A section of land where rain or snow water drains downhill into a body of water, such as a river, lake, estuary, sea or ocean. 2. An area from which a community/region receives its water supply.

Snowpack one source of headwaters (melting snow independent of precipitation)

Another source of headwaters Bottom left

Watershed Divide hills and ridges that form the boundary of the watershed

Where is YOUR watershed?

A Healthy (natural) Watershed

An Unhealthy Watershed (when human development doesn’t work WITH nature) 1. Buildings and pavement are impervious surfaces, preventing water from infiltrating the ground and creating too much surface runoff. 2. Clear-cut hillsides allow erosion which could lead to the siltation of streams. 3. Heavy agricultural wastes send excessive nutrients into the waterways which ultimately produce dead zones in the ocean. 4. Septic tanks can leak and send human wastes into the groundwater.

A miniature watershed seen at Salmon Days

And our own creation

Wildlife components: A black bear hunting for its prey.

This one caught a coho salmon. Glaucous-wing gulls hope for scraps.

Salmon heading up tributaries to their spawning ground.

An estuary: where rivers slowly blend into the sea.

Since the estuary is an ideal spawning ground for many fish, you will find many predators here

Great blue heron spearing its prey

Stormwater runoff, because of an impervious surface

The next slides are about topographic maps (not on the watershed test)

Reading a Topographic Map

1. Issaquah’s old air strip