Anatomy of a Drying River

I have, on more than one occasion, been met with a little derision by people from more wetter parts of the country when I discuss southern Californian rivers. What constitutes a "river" here may for most of the year be little more than a stream or actually completely dry up. Never the less in certain water starved parts a very generous definition of river suffices. This ephemeral nature of many of our rivers has much to do with human use such as wells, diversions, and dams. But anyone who goes out and hikes into our watersheds should be familiar with creeks that show good flow but then disappear into the earth and then often times reappear later on downstream. As a child I was always puzzled by this; where did the water go? And then, with no obvious signs of surface input, why did the water sometimes reappear downstream?

It was not until I understood the three dimensional nature of water movement in our landscape that I came to understand this pattern better for what it is. Our streams and rivers, ephemeral as they may be, offer a view into water dynamics that those from wetter parts never fully get to realize.

The first useful concept to visualize is groundwater. Groundwater is a zone of saturation that lies underneath in the earth at varying depths but more or less follows the contours of the land. Wells tap into this. Ponds, rivers, lakes, marshes and other bodies of water represent the interface between the water table and the surface.

USGS (c)

Seen three dimensionally it can be seen here how, even with no obvious signs of surface recharge,  a stream can flow all year long with adequate recharge from groundwater. Such a a stream would be characterized as a "gaining" river. In our ephemeral streams this would represent the reappearance of water after a dry part or a river that flows year round. 

But what is also crucial to dynamic watersheds is an understanding of aquifers. Aquifers are large subterranean bodies of water, often confined by certain layers of impervious sedimentary rocks. Many arid regions depend on withdrawing from aquifers for irrigation purposes. When these aquifers become depleted of water they want to suck it back up, either from the water table/rivers or even from the ocean (saltwater intrusion). When the aquifer is hungry it will lower the water table and deplete above ground water resources. You now have a "losing" stream and this is where rivers/streams dry up.

USGS (c)

Seen in this manner you can visualize the above ground portion of the water table- streams, rivers, lakes etc. etc- as representing a dynamic equilibrium between above and below ground water flow. Where underground aquifers are happy, above ground flow is good and the water table is high. Where aquifers are thirsty the water table is low and above ground flow is minimal.

And at this point in our discussion, if you have been connecting the dots, it should start to crystallize in your skulls how beaver- with their ability to store water in dams and recharge aquifers slowly through the year- can transform a "losing" stream system into a "gaining" one. And this ability of beavers to create a "well watered land" is especially pivotal in arid regions.

Immature Black-Crowned Night Heron. Oxnard CA Steve Nash (c)