What is a watershed?

Watersheds of the Willamette River BasinWhen we talk about a water­shed on these pages, we’re refer­ring to the land area drained by a river or stream.

So a “water­shed” can be the huge land area drained by the Colum­bia River or the area drained by the Willamette River.  A water­shed can be the 766 square miles drained by the North San­tiam River or the few hun­dred acres drained by a small creek.  Regard­less of size, they all drain an area of land or “watershed.”

And just like that set of mea­sur­ing cups you pull out when you’re get­ting ready to bake cook­ies, water­sheds come in dif­fer­ent sizes and just like the mea­sures, they come nested, one in the other.

Take the exam­ples I used. The Colum­bia River is the largest and the Willamette River flows into the Colum­bia at Port­land.  That means the Willamette River’s water­shed is nested inside the Colum­bia River water­shed, just like my ½ cup mea­sure nests inside the full cup one.

Watersheds are nested, like measuring cups

Now here’s where the mea­sur­ing cup exam­ple breaks down, because only a sin­gle ¼ cup mea­sure nests inside the ½ cup one. But when it comes to water­sheds, a bunch of smaller water­sheds fit into the larger ones.  For exam­ple, the North San­tiam water­shed is one of many com­po­nent water­sheds of the Willamette River’s water­shed.  In fact, each of the trib­u­tary rivers, streams and creeks that flow into the Willamette have a water­shed that is nested in the larger Willamette River’s. So, a water­shed can encom­pass mil­lions of acres like the Colum­bia River Basin, or a few acres for a sin­gle small stream.  They come nested, with more than one smaller water­shed per pack.

Other terms used for water­shed are drainage basin, catch­ment, catch­ment area, catch­ment basin, drainage area, river basin and water basin.  To make things even more con­fus­ing, in the United King­dom and Aus­tralia, a water­shed refers to a divide or ridge that sep­a­rates one water­shed from another.

If you’re a lit­tle con­fused, don’t worry you’re in good com­pany. Sci­en­tists have been try­ing to come up with one uni­ver­sally agreed on, rea­son­able, descrip­tive way to name and clas­sify water­sheds or for some time and haven’t really suc­ceeded yet.

But let’s agree, at least here on our pages where we’re try­ing to make mat­ters easy to talk about, we’ll use the terms basin and water­shed inter­change­ably when we refer to the entire North San­tiam drainage area.  But we will use the term water­shed, as in “the Mad Creek water­shed” when we refer to the smaller com­po­nent drainages of the North San­tiam Basin.  And we’ll name our water­sheds for the creek or river that drains the area.

By the way, you can find out more about clas­si­fy­ing and nam­ing water­sheds at: Wikipedia or from this doc­u­ment that explains the clas­si­fi­ca­tion of water­sheds and other hydro­logic units that’s pub­lished by the US Depart­ment of Agriculture.

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