The Evolution of Drainage

Drainage Evolution

Beveling Topography 

Fluvial landscapes evolve over time.

Imagine a place where continental collision uplifts a region; the landscape will evolve (figure above a). At first, rivers have steep gradients, flow over many rapids and waterfalls, and cut deep valleys. But with time, rugged mountains become low, rounded hills; once-deep, narrow valleys broaden into wide floodplains with more gradual gradients. As more time passes, even the low hills are beveled down, becoming small mounds or even disappearing altogether. (Some geologists have referred to the resulting landscape as a peneplain, from the Latin paene, which means almost; it lies at an elevation close to that of a stream’s base level.) 

Though the above model makes intuitive sense, it is an oversimplification. Plate tectonics can uplift the land again, and/or global sea-level rise or fall can change the base level, so in reality peneplains rarely develop before downcutting begins again. Stream rejuvenation occurs when a stream starts to downcut into a land surface whose elevation lies near the stream’s base level. Rejuvenation can be triggered by several phenomena, such as: a drop in base level, as happens when sea level falls; an uplift event that causes the land to rise relative to the base level; or an increase in stream discharge that makes the stream more able to erode and transport sediment. As we've seen, rejuvenation can lead to formation of stream terraces in alluvium. In cases where rejuvenation causes a stream to erode deeply into the land, a new canyon or valley will develop. If the rejuvenated stream had a meandering course, downcutting produces incised meanders (figure above b). 

Stream Piracy and Drainage Reversal 

The concept of stream capture or “piracy.”

Stream piracy sounds like pretty violent stuff. In reality, it’s just a natural process that happens when headward erosion by one stream causes the stream to intersect the course of another stream. When this happens, the pirate stream “captures” the water of the stream that it has intersected, so that the water of the captured stream starts to flow down the pirate stream. Because of piracy, the channel of the captured stream, downstream of the point of capture, dries up (figure above a, b). In some cases, stream capture changes a “water gap” (a stream carved notch through a ridge) into a dry “wind gap.” In 1775, Daniel Boone, the legendary pioneer, led settlers through the Cumberland Gap, a wind gap in the Appalachians, to new homesteads in western Kentucky. 

The pattern of stream flow in an area can also be altered, over time, on a continental scale. For example, when South America and Africa were adjacent to each other in Pangaea, a highland existed along the boundary between the two continents, and the main drainage network of northern South America flowed westward. Later, when South America rifted away from Africa, a convergent plate boundary developed along the western margin of South America, causing the Andes Mountains to rise. The uplift of the Andes caused a drainage reversal, in that the overall slope direction of the drainage network became the opposite of what it once had been. As a consequence, westward flow became impossible, and the eastward-flowing Amazon drainage network developed.

Superposed and Antecedent Streams 

The structure and topography of the landscape do not always appear to control the path, or course, of a stream. For example, imagine a stream that carves a deep canyon straight across a strong mountain ridge why didn't the stream find a way around the ridge? We distinguish two types of streams that cut across resistant topographic highs: 

 Formation of superposed drainage

• Superposed streams: Imagine a region in which streams start to flow over horizontal beds of strata that unconformably overlie folded strata. When the streams eventually erode down through the unconformity and start to downcut into the folded strata, they maintain their earlier course, ignoring the structure of the folded strata. Geologists call such streams superposed streams, because their pre-existing geometry has been laid down on underlying rock structure (figure above a, b). 
• Antecedent streams: In some cases, tectonic activity (such as subduction or collision) causes a mountain range to rise up beneath an already established stream. If the stream downcuts as fast as the range rises, it can maintain its course and will cut right across the range. Geologists call such streams antecedent streams (from the Greek ante, meaning before) to emphasize that they existed before the range uplifted. Note that if the range rises faster than the stream downcuts, the new highlands divert (change) the stream’s course so that it flows parallel to the range face (figure below a–c).

 Development of antecedent and diverted streams.

Credits: Stephen Marshak (Essentials of Geology)

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