Water does move the world, but not always in a noticeable or dramatic way. Erosion is a key element in making the
world possible to live in. Erosion forms the valleys, literally dissolves solid rock, moves 3 ton boulders, and builds
bridges. Water is the most abundant and therefore most powerful resource on the planet. Although water creates
landmarks the size of the Grand Canyon, man cannot see the process, only the result.
According to Rain Erosion: graphics.lcs.mit.edu, rain erosion is
a result of kinetic energy developed by water as it flows over the surface of the
material. Material particles are detached from the mass and transported to
another location.
Although one raindrop has little effect on the earth, the accumulated effect of rainfall over long periods of time can
accomplish large amounts of erosion.
Erosion begins when rainfall hits materials traveling at a high velocity and forces the materials to crumble into small
particles. This continues until the rain creates small grooves. Continuing rain causes these grooves to overflow with
a combination of water and particles. This is called run-off. Continuous run-off breaks up the surface into smaller
and smaller particles. Accumulating run-off on the surfaces moves down the slopes. This causes sheet erosion. This
down slope run-off detaches these particles and moves them with the water. These moving particles strike against
other particles on the surface which sets them into motion: this process is called abrasion. The velocity and
turbulence of the run-off affect the degree of the sheet erosion; also some materials are less abrasive than others
which also affects the degree of sheet erosion.
Matter can be transported in three different ways: heavier particles will roll along the surface--these are called
rolling matter. Smaller particles will bounce along at a faster rate than rolling matter----these are called bouncing
matter. The smallest particles are completely suspended by the water and travel the fastest-----these are called
suspended matter.
The particles comes to rest when resistance forces are greater than
the velocity of the run-off.
There are three types of river erosion: chemical erosion; hydraulic erosion, and abrasion. Chemical erosion occurs
when chemicals in the water react with minerals in the surface materials and cause them to dissolve and break apart;
the dissolution can cause chemical compounds to form such as salts. The salts are carried in the form of ions. Rock
gypsum, for example, contains the compound Calcium Sulfate. When this is dissolved in water it creates calcium
ions and sulfate ions without several other salt compounds. It does not effect the taste of the water.
Hydraulic erosion is based completely on the force of the moving water. A rapidly moving stream can widen cracks
and break off large chunks of fractured rock. According to Lakes and Streams by Laurence Pringle, . . .in 1923 a
stream at flood stage in the Wassatech Mountains of the northwestern United States wrenched loose boulders
weighing up to 90 tons and carried them over a mile downstream. Abrasion is the third and most powerful form of
river erosion.

A young valley is usually a gully or ravine on a mountainside. It will usually zigzag and have steep sides. The
bottom is usually rock with potholes and boulders. The brooks have many waterfalls, rapids and pools. Downstream
from a young valley there is usually an older, or mature valley. Mature valleyıs streams are longer, straighter and
wider, due to years of erosion. They have a smooth bottom liminating rapids and pools. Down sloping in the valley
has reduced the steepness of the slopes, therefore slowing down the flow of the stream and slowing also the process
of erosion---so the bigger a valley gets, the longer it takes to get any bigger.
Mature valleys are much wider than they are deep. The stream now runs in a narrow channel on the valley floor.
Years of erosion have transformed boulders and cobble into sand and gravel . Old valleys are extremely wide
compared to their depth. The stream flows slowly through a winding channel on the valley floor. The stream is now
so slow that erosion is scarce. The stream work consists mainly of transportation and deposition.
An extremely old valley can be miles wide and only a few yards deep. The walls can still be as steep as they