>>Today we will discuss deltas. Deltas are the end of the line, where a river enters
a large body of water. At this location you'll get deposition--it's called progradation where
we add land. Why do you get deposition when a river enters a large body of water? Well,
the velocity decreases, that reduces the competence of the river and you get deposition. There
are three major processes that operate on most marine deltas. And these are: fluvial
or river processes, which bring the sediment to the delta; waves and tides, which redistribute
the sediment once the rivers deliver the material to the delta. We're going to talk now about
a fluvially-dominated delta, like the Mississippi Delta. What are the environments that occur
on a fluvially-dominated delta? They include the distributary channels, which are basically
the river; the distributary mouth bars (or DMBs)--that's the place where the river actually
enters the ocean and consists mostly of sand. There are levees at the edges of the distributary
channels, which are produced by flooding, and are little mounds that make the river
channel bigger. There are delta front sheet sands, which are caused by principally waves,
which distribute the sand that has been delivered to the DMB. There's also the interdistributary
bay between the distributary channels, which includes things like swamps and large lakes,
and then there's the prodelta. The prodelta is out in front of the delta, and this is
where you get most of the deposition of fine material, like clay. Look at the virtual field
trip to the Mississippi Delta to get a feel for the different environments and processes
that operate on a delta. There are several important processes that operate when a river
enters the ocean and a delta is produced. First, sand is deposited near the mouth of
the delta at the distributary mouth bar. Clay is deposited farther out in the prodelta.
Why do you get so much clay deposition in front of a delta? It's because of something
called flocculation. The clays when they enter the ocean water clump together, they get larger
so they're deposited. And there's several stages in delta development that we're going
to talk about. And these are very important. First, you start off and you get progradation,
you add land. And I'm going to use my arm here as the shoreline and as the delta builds
out--here's the ocean, here's the delta, it's building out--you get progradation. Okay,
that's the first stage. But what's happening here? You're getting sand deposition on top
of what? Mud and shale, and what happens to mud and shale when you put sand on top of
it? It sinks. You get compaction. So long as you have deposition, you're still going
to get progradation, but you're always getting compaction, and that's very important on a
delta. Now eventually this delta's going to build out so far into the ocean that what's
going to happen? It's going to slow down, the river will slow down because the gradient
has decreased. Because what is sea level? It's a flat gradient. So if you think about
it, there's a much shorter path to the ocean from my elbow to the ocean than there is following
this delta path. So what happens? A new delta forms by a process called avulsion. The river
switches its course. It seeks a shorter path to the ocean and builds a delta out over here.
What happens to the old delta? Well, if you're no longer delivering sediment, remember we
have compaction and subsidence, so it's going to sink. But a new delta builds out over here.
The Mississippi River has switched about seven times in the last 5,000 years, and we're going
to talk about that in a little bit more detail in the next video.