Running Water Causes Erosion
OF ALL OF THE AGENTS OF EROSION, RUNNING WATER IS THE MOST POWERFUL.
For our purposes running water includes everything from precipitation like rain and snow, streams, and rivers.
When rain falls on the Earth’s surface it will always move to the lowest elevation possible. Usually these trickles of rain water combine to form larger streams which empty into even larger rivers, and eventually into a lake or ocean.
Running Water Transports Sediment
As each of these forms of running water moves over the land it picks up and carries weathered rock material. Running water can carry these weathered rock pieces in three different ways: in solution, suspension, and as bed load.
MATERIALS THAT ARE CARRIED IN SOLUTION ARE DISSOLVED IN THE WATER. THE SMALLEST PARTICLES, MINERALS, ARE CARRIED IN SOLUTION.
They are mostly minerals that are dissolved out of the bedrock – most commonly calcium and magnesium compounds. These dissolved particles are so small they can't be seen with the naked eye.
Larger particles of weathered rock cannot be dissolved into the water. These particles are carried along in supsension.
SUSPENDED PARTICLES, LIKE SAND, SILT, AND CLAY, ARE PUSHED ALONG BY THE CURRENT IN THE MIDDLE OF THE STREAM. THEY ARE LIGHT ENOUGH THAT THE CURRENT CAN KEEP THEM OFF THE BOTTOM, BUT TOO HEAVY TO FLOAT.
Even though they are heavier than water, the force of the moving water keeps them from sinking and carries them in the middle of the water.
BED LOAD IS THE LARGEST, DENSEST PARTICLES CARRIED IN A RIVER BY TUMBLING ALONG THE BOTTOM.
Large sand grains, pebbles, and boulders, are even heavier rock particles, so they can’t be kept afloat by most rivers. Since they can’t be kept up in the moving water, they just get pushed along the bottom as the bed load of the river.
US geologists estimate that our rivers carry about 25% of their rock material in solution, another 50% in suspension, and another 25% as bed load
Rocks Carried in Running Water Change
WEATHERED ROCK CARRIED IN STREAMS HELPS TO BREAK APART OTHER ROCKS.
The tiny pieces of weathered rock are constantly being smashed into each other, and into the rocks on the sides and bottom of the river. When the slam into other rocks like that they act like little tiny cutting tools by smashing off tiny pieces of the other rock.
ABRASION IS THE GRINDING ACTION THAT OCCURS AS SEDIMENTS CARRIED BY RIVERS SMASH INTO EACH OTHER BECOMING SMALLER AND ROUNDER.
Abrasion not only carves away at stream beds, but also causes the rocks in rivers themselves to be worn down. Over time rocks carried in rivers become smaller, smoother, and rounder. The longer a rock is carried within a river, the rounder and smoother it will become.
Abrasion does most of the weathering that occurs in running water, but the lifting effect of flowing water also causes rocks to split up and break. Running water over exposed bedrock also is constantly causing chemical weathering as the water dissolves the minerals of the bedrock.