but then it settles back down to the stream bed because it is too heavy to remain suspended
in the water. This is calledsaltation,which we will learn about later in this chapter with
transport of particles by wind. Streams with high velocities and steep gradients do a great
deal of downcutting into the stream bed, which is primarily accomplished by movement of
particles that make up the bed load. Particles that move along as the bed load of a stream
do not move continuously along, but rather in small steps or jumps with periods of remaining
stationary in between.
Stream and River Erosion – Stages of Streams
As a stream moves water from high elevations, like mountains, towards low elevations, like
the ocean, which is at sea level, the work of the stream changes. At high elevations, streams
are just beginning streams that have small channels and steep gradients. This means that
the stream will have a high velocity and will do lots of work eroding its stream bed. The
higher the elevation, the farther the stream is from where it eventually meets the sea.Base
levelis the term for where a stream meets sea level or standing water, like a lake or the
ocean. Streams will work to downcut their stream beds until they reach base level.
As a stream moves out of high mountainous areas into lower areas closer to sea level, the
stream is closer to its base level and does more work eroding the edges of its banks than
downcutting into its stream bed. At some point in most streams, there are curves or bends
in the stream channel calledmeanders(Figure10.3). The stream erodes material along
its outer banks and deposits material along the inside curves of a meander as it flows to
the ocean (Figure10.4). This causes these meanders to migrate laterally over time. The
erosion of the outside edge of the stream’s banks begins the work of carving afloodplain,
which is a flat level area surrounding the stream channel.
Stream and River Deposition
Once a stream nears the ocean, it is very close to itsbase leveland now deposits more
materials than it erodes. As you just learned, one place where a river deposits material is
along the inside edges of meanders. If you ever decide to pan for gold or look for artifacts
from an older town or civilization, you will sift through these deposits. Gold is one of the
densest elements on Earth. Streams are lazy and never want to carry more materials than
absolutely necessary. It will drop off the heaviest and largest particles first, that is why you
might find gold in a stream deposit. Imagine that you had to carry all that you would need
for a week as you walked many kilometers. At first you might not mind the weight of what
you are carrying at all, but as you get tired, you will look to drop off the heaviest things you
are carrying first!
When a river floods or overflows its channel, the area where the stream flows is suddenly
much broader and shallower than it was when it was in its channel. This slows down the