Friday, November 07, 2014

River valleys

1. Which comes first–the river or the valley (ravine, canyon)? I'm not a geologist or hydrologist, but I believe this can happen in at least one of two different ways. Here's a conventional explanation of one process:

The true creator of a canyon is water, primarily in the form of a river. Over millions of years, water has scoured and cut away layer upon layer of rock, lowering a canyon's floor and widening its walls.
Others have been carved through multiple layers of igneous rock, which is formed by the cooling and hardening of magma, melted rock material from within Earth, and metamorphic rock, whose texture or composition has been changed by extreme heat and pressure.
Slot canyons are cut and scoured by rushing water in the form of flash floods. A flash flood is a flood that occurs after a period of heavy rain, usually within six hours of the rain event. In arid environments where there is little soil to absorb the rain, water quickly runs downhill, gathering volume and speed as it goes. When it runs over the canyon, it descends in a wall of water that blasts through the canyon, eroding the walls and floor. As quickly as the water appears, it disappears, leaving the canyon dry and slightly changed until the next flood.
Water is a natural force of erosion everywhere on Earth. Surging over a landscape, water will pick up and transport as much material from the surface as it can carry. Aided by gravity and steep slopes, rushing water can carry increasingly larger and heavier objects, including boulders as large as cars. If a river and its surroundings have been elevated from their original position by natural forces within the planet, that river will seek to return to its natural level as quickly as possible. Finding the least resistant path, a river will cut through rock layers. Lowering its floor little by little, the river will take millions of years to carve through the surrounding rock before it reaches the level it seeks. In the process, it creates a canyon.
The rivers that created the canyons on the Colorado Plateau and elsewhere did so because rivers have a natural tendency to reach a base level. This refers to the point at which the river reaches the elevation of the large body of water, such a lake or ocean, into which it drains. Aided by gravity, a river will downcut or erode its channel deeper and deeper in order to reach the level of its final destination as quickly as possible. The larger the difference in height between the river and its destination, the greater the erosive or cutting force of the river.
Rivers erode because they have the ability to pick up sediments (loose rock fragments) and transport them to a new location. The size of the material that can be transported depends on the velocity, or speed, of the river. A fast-moving river carries more sediment and larger material than a slow-moving one. As it is carried along, the sediment acts as an abrasive, scouring and eating away at the banks and bed of the river. The river then picks up this newly eroded material, which, in turn, helps the river cut even deeper into its channel.
If a river cuts through resistant rock, such as granite, its channel and the canyon it creates will be narrow and deep. If it cuts through weaker material, such as clay or sandstone, its channel and its accompanying canyon will be wide. When cutting through soft rock, a river can undercut its banks, removing a soft layer of material while a harder layer remains above, forming an overhang. The overhang continues to grow as material beneath it is eroded away by the river until the overhang can no longer be supported and collapses into the river. Repeated undercutting can lead to landslides and slumps, creating a V-shaped canyon.

i) How long this naturally takes depends on a variety of factors. How hard or soft the layers are. The volume and rate of runoff. 

ii) I'm also guessing that lava flows can rapidly create river channels. 

2. On this model, the river comes first. The valley (ravine, canyon) is the result of erosion from runoff. 

But I assume the principle can operate in reverse. If there's a preexisting valley (ravine, canyon), then that's the route that runoff will take. That will channel or funnel runoff. On that model, the valley (ravine, canyon) comes first. The river course is the result of that preexisting topography. 

3. In principle, these can be complementary dynamics. Preexisting topography might create a natural drainage outlet for runoff. Conversely, runoff will deepen and widen the drainage outlet.

4. This has potential implications for young-earth creationism. Can you tell, just by looking at a river valley, which came first–the river or the valley. What was the mechanism? 

5. Young-earth creationism has two different explanations:

i) Flood geology attributes some canyons to a global deluge. 

ii) However, young-earth creationism can also attribute some valleys, ravines, canyons, &c. to mature creation. God made the world with a preexisting topography of some sort. That could include built-in drainage outlets for runoff.

iii) It may be difficult to sort out which is which this far down the pike. Is an extant valley (ravine, canyon) the result of mature creation, Noah's flood, or normal processes? For instance, I assume a volcanic eruption or massive earthquake might create new river channels. Likewise, a depression that's the result of mature creation will widen and deepen over time due to continuous erosion. Or so I imagine. I'm no expert. 

Of course, there's the complication of conventional dating methods. 

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