Watersheds - The Water Flow

Watersheds define an area of land and their overall shape and size determines the amount of water in a watershed.  Watersheds funnel flow to a single point, usually a path to the ocean.  A watershed map shows how water flows over a specific area of the globe.  The largest watershed in the world, the St. Lawrence/Great Lakes watershed contains 20% of the world's fresh water.

Because of shape, watersheds vary from thousands of miles long to just a few hundred yards.  The Mississippi River watershed stretches from Idaho to Pennsylvania and from Minnesota to Louisiana while draining parts of 31 states and two Canadian provinces.

All water constantly seeks lower ground.  Some mistakenly think that water is directional, as in it follows the compass.  Actually water flows from high to low.  Water flows from high elevation (feet of sea level) to lower elevation.  The Mississippi only drops a few hundred feet of elevation along it's entire 2000-mile path.  Yet, short creeks along the California coast can easily drop over 2000 feet.  By observation, the faster water flows, the steeper it's path.  Follow water upstream to find the source and downstream to find the mouth.

Water starts up high.  Often, the cleanest, coolest, and clearest water resides up high, in the mountains.  For all water flows, brooks, creeks and rivers, the water stream begins somewhere and the smallest flows are at the highest elevations.  Watersheds collect water over a wide area so their headwaters come from all directions and the watershed gathers all these sources together into one.

Once gathered, into a creek for example, a water flow terminates somewhere.  Most fresh water returns to the ocean.  However, some water also flows into inland water bodies, terminal lakes, like the Great Salt Lake.  The Great Basin in the United States traps water into inland water bodies throughout the west.  Not only does the Great Basin encompass the Great Salt Lake but also Lake Tahoe and the Salton Sea.  Hydrologists call these watersheds endorheic basins, but HydroFathom calls the water bodies terminal lakes.  Some precipitation soaks into the ground and HydroFathom cover this water under our Aquifer section.  Otherwise, all other water flows will eventually touch the oceans, the ultimate source of water for all living things on land.

Obviously, the amount of rain and snow (precipitation) that falls on a watershed determines how much water flows out of a watershed.  At Hydrofathom, we often examine the affect of 1" of rain falling on a watershed.  For example, The watershed of our hometown, San Luis Obispo, is about 85 sq. miles.  If just one inch of rain falls across that entire watershed, then 200 millions of gallons of water falls from the sky.  Learn the watershed for your area and try it.