River Physics

river.0Rivers meander in much the same way that snakes slither—their forward movement is not only lateral but also longitudinal. A time-lapse map of a river’s channel is like the revealing of some great, ancient secret. The simple movement of water through the landscape is one of the most powerful forces on Earth.

Rivers and streams are fascinating, deeply complex creatures. They are hugely influential ecological pathways, contributing to essential ecosystem processes both above and below the surface. The critical watershed-scale work of the river is to transport a balance of water and sediment downstream to the receiving water source (ocean, lake, bay, et cetera). The ecosystem processes that spring from that single task are enormous in scope and are critical to a large majority of California’s wildlife.

The geomorphic characteristics of watersheds change dramatically from their headwaters to the river mouth (geo = earth, morph = shape, form). The word “geomorphology” describes the behavior of the physical structure of rivers and the study of that structure. The upper reaches of streams are composed of large boulders and very steep slopes, and are tightly confined within their valleys. The action of water on this rock, the river’s substrate, is partially responsible for the creation of the sediment-water balance. Because of the steepness of these mountain streams, their channels are typified by a step-pool structure with very little meandering. This is the reason that mountain streams are clear, clean, and unusually musical.

Where these steep valleys open up, a new source of sediment joins the river—bank sediments created by the natural erosive process of meandering. Rivers that are allowed to move freely within their meander belts are continuously eroding and rebuilding their meanders. A river can meander while still remaining confined to its valley because, in its snakey movement, erosive energy is transferred downstream as well as laterally.

flickr_looping_amazon_river_2These middle areas of the watershed both collect and deposit sediments; they are deposited in the form of point bars, which form the inside of a meander. A well-sorted point bar, with fine sediments higher on the bar and progressively coarser sediments toward the active channel, is one sign of a healthy river. This process occurs because the slower water is moving, the more sediment it will drop out of suspension. When water slows down on the high edge of a point bar, the sands and silts drop out; in the thalweg (deepest point of the channel) fast water will roll larger bed material downstream while maintaining its suspended sediment load. Rivers in equilibrium will not excessively erode nor aggrade their sediment. Rivers out of equilibrium—well, we will hear much more about those later.

Finally, in the lower reaches, rivers begin to branch out into tidal marsh complexes typified by very fine, rich sediments and braided channels. Nearly the entire San Francisco Bay was once bordered by vast expanses of tidal marsh. Tidal marshes provide both critical ecosystem processes and benefits to human communities, including nutrient and pollutant filtration, carbon sequestration, and flood mitigation. Intact marsh complexes actually protect coastal communities from storm surges—a fact powerfully demonstrated by Hurricane Katrina.

This incredible process creates beaches and nourishes tidal marshes and the biotic community that relies on riparian corridors—not only in California, but in watersheds around the world. There is something about the universality of river physics that is both electrifying and oddly comforting—that even if the river is damaged, the stream channelized, the energy is still out there somewhere, waiting for its time to go to work once again.

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Credit to Flickr: leoffritas (Creative Commons 2.0) and @josephstromberg at Vox for the super cool imagery.

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