Milestone project on geologic analysis

PHY 103 Module Two 1

In Module One, rocks, minerals, and the rock cycle were examined; Module Two explores

fluvial processes. Running water is the most dominant agent of landscape alteration on

Earth. Most of the landscapes on Earth have been fashioned by water, even in

predominantly dry locations. Module Two explores how water shapes Earth’s surface.

The hydrologic cycle describes the unending circulation of Earth’s water supply. This is the

primary link between the oceans (which store almost all of Earth’s water) and the continents.

As such, the cycle links all water sources through evaporation, condensation, precipitation,

runoff, groundwater, soil water, streams, lakes, oceans, the atmosphere, and ice. Here, we

specifically focus on surface water (runoff) and streams.

A stream is a channel flow of any size. Large streams that have tributaries (small streams

that contribute water to the main stream) are called rivers. Streams are supplied with water

in two ways: runoff and groundwater. Of these, groundwater makes up the bulk water supply

on average. This is why streams do not dry up if rain does not occur for a while.

Groundwater contributes a baseflow to streams. Baseflow is the lowest level a stream will

drop to under dry conditions. Streams are comprised of water, which is flowing (usually) to

the sea. The time required for water to reach the sea largely depends on stream velocity

(speed), which itself is determined by the stream gradient. Gradient is the slope of a stream

channel.

Automatically one would assume that the high-gradient area near the stream source (or

head) would have a higher stream velocity. That is true, partly, but streams normally see the

highest average velocities in low-gradient areas near the mouth. This is because velocity is

also determined by discharge (the total volume of water in a stream). Discharge is much

higher near the mouth than the source as water is added along the entirety of the stream

path. A good example of this is the Mississippi River. Near the source in Minnesota, the river

is small and shallow, but seemingly moving rapidly. Near the mouth, the river is over a mile

wide with a discharge of well over half a million cfs (cubic feet per second). Although the

river looks slow-moving (and many have dubbed it the “lazy Mississippi”), the impressive

discharge indicates that the water is moving very quickly.

2 PHY 103 Module Two

There are several terms to be aware of when discussing streams. The longitudinal profile

of a stream is a cross section of the stream from source to mouth. Using this, it is easy to

see how and why discharge is inversely related to gradient. Base level is the lowest point to

which a stream may erode its channel and the surrounding landscape. This is normally sea

level (ultimate base level). If the stream erodes past sea level, the landscape simply

becomes part of the sea itself. Temporary base levels may also exist. These are lakes or

reservoirs or even other streams in the stream network (all streams that are connected).

Streams erode their own beds in a process called downcutting. As the stream incises into a

landscape, the gradient of the surrounding landscape increases. This triggers an increase in

the velocity of water moving across the landscape into the stream, itself causing higher rates

of erosion. Over time the entire landscape reduces in elevation.

Eroded materials in streams are called sediments. They can exist as very large sediments

pushed along the bottom of the stream (the bed load), suspended (bulk load) sediments, or

in solution (dissolved load). When a stream reaches a base level, the gradient reduces to

zero. At this point the stream energy decreases dramatically and sediments are deposited.

Typically the largest particles are deposited first, near the stream mouth, while finer particles

are carried farther out. A new landscape at the mouth may develop with distinct sedimentary

beds. This landscape is called a delta. Deltas form differently in relation to different river and

ocean associations. Where currents are swift, alluvium (river-deposited sediments) are

smoothed at the coast, creating a classic delta (the Greek letter) shape. An example is the

Nile River Delta in Egypt. Where currents are slow-moving, each distributary (small rivers

that drain from the main river) form their own small deltas. An example of this is the “bird’s

foot” delta of the Mississippi River in Louisiana.

Upstream, two types of valleys occur from associated stream erosion. V-shaped valleys

occur where gradients are high and stream energy is used for downcutting. The valleys have

a V shape profile with little or no floodplains and steep sides. Rapids and waterfalls are

typical and form over areas with highly resistant (to erosion) bedrock. Niagara Falls and the

Niagara Rapids in the Niagara River are good examples.

In areas of reduced gradient, stream energy will be used to erode the surrounding

landscape. This lateral erosion causes the development of river meanders (bends), which

widen the floodplain over time. This results in a wide-flat floodplain that can be either an

erosional floodplain or a depositional floodplain depending on the factors involved.

Common features of a wide-flat floodplain include meanders that have high rates of water

flow on the outside of the meanders (the cut-bank). Cut-banks are the erosional areas and

result from centrifugal force in water moving around the curve. The inside of the meanders

(point bars) will have slow water movement, which promotes deposition. Meanders may

PHY 103 Module Two 3

grow to huge proportions and sometimes become cut off. The cut-offs create oxbow lakes

(abandoned meanders), which eventually fill in to form meander scars on the floodplain.

Back swamps, natural levees, and Yazoo streams (small streams that drain the back

swamps and join the river as a tributary) also commonly form in wide floodplains. Yazoo

streams are named after the Yazoo River in Mississippi. It parallels the Mississippi River for

a long distance before joining as a tributary.

Over time, all landscapes reduce from high gradients to low gradients through stream

erosion. Thus, valley development proceeds from youth (V-shaped valleys) to mature

(development of a wide erosional floodplain) to old age (a very wide depositional floodplain).

In some cases, tectonic forces or changing sea levels may cause rejuvenation.

Rejuvenation is when a wide, flat depositional floodplain river finds itself high above base

level. Downcutting is promoted in the old-age, meandering stream. The stream course does

not change, but incising of the landscape occurs, leading to meandering canyons. The

Grand Canyon is a prime example of rejuvenation.

Groundwater is also important, as it is the primary source of the dissolved load in streams

and the source of baseflow. Groundwater can cause distinct geologic features, such as

caves, sinkholes, and disappearing streams, and cave features such as stalactites and

stalagmites. Other groundwater-derived features link tectonic activity (esp. magma) to

produce hot springs and geysers. Also, aquifers (rock layers that can transmit water) may

produce artesian wells (those that are under pressure and require no pumping) and non-

artesian wells (pumping required) because the aquifers are inclined and trapped between

aquitards (rock layers that do not transmit water).

In conclusion, Module Two examines fluvial processes and their impact on landscape

formations. Module Three continues with glaciers and glacial formations.