Chapter

Stream erosion and deposition are controlled primarily by a river's

Velocity

Drainage patterns

a) Dendritic b) Radial c) Rectangular d) Trellis

Name and describe the three main ways in which a stream transports sediment.

1) Bedload (particles moved along the bottom of the stream) 2) Suspended load (particles remaining in suspension above the bottom of the stream as they're carried in the water) 3) Dissolved load (chemical constituents in solution).

What factors control a stream's velocity?

1) Gradient, or slope of the stream. 2) Discharge, or volume of water in the stream. Generally, as either the slope or the discharge of a stream increases, the velocity increases. 3) Stream channel shape, the shape of the bed of rock or loose materials that confine the stream.

Describe three ways in which a river erodes its channel.

1) Hydraulic action (erosion due to the force of moving water plucking out bedrock from the sides and bottom of the river) 2) Abrasion (grinding away of the stream channel by friction from sediment) 3) Solution (dissolving away of soluble mineral constituents such as calcite).

What is the gradient of a stream that drops 10 vertical feet over a 2-mile horizontal distance?

5 feet per mile

A platform of sediment formed where a stream flows into standing water is

A delta

The broad strip of land built up by sedimentation on either side of a stream channel is

A flood plain

How does a natural levee form?

A natural levee forms due to a sharp decrease in water velocity as a stream at bankfull discharge overtops its bank in a particular spot. With the water no longer confined to a narrow channel, water velocity decreases. As velocity decreases, much of the sediment in the escaping water is deposited along the fringe of the main channel, creating a natural levee.

What does a trellis drainage pattern tell about the rocks underneath it?

A trellis pattern indicates that the underlying rocks are tilted and the rocks alternate between resistant units (ex: sandstone, volcanic rock) and less resistant units (ex: shale).

How does a meander neck cutoff form an oxbow lake?

After a long period of time, the meander becomes very curved, and eventually the neck of the meander becomes narrower and the river cuts through the neck during a flood, cutting off the meander and forming an oxbow lake.

Give at least four arguments against building dams on rivers. Give at least four arguments in favor of building dams.

Arguments in favor of building dams: - Renewable clean power generation - Flood control - Irrigation - Storage helps maintain a higher river flow in late summer and during droughts - Recreation/boating on reservoir Arguments against building dams: - Reservoir submerges wildlife habitat - Dam releases colder water, which may affect native fish species - May reduce whitewater recreation on river - May impact fish migration

Describe how bar deposition creates a braided stream.

As sediment is deposited in the bed of the stream, a mid-channel bar is eventually created, which in turn diverts the water in the stream around the bar. As multiple bars divert water along various pathways, a braided pattern is created.

Cobbles are more likely to be transported in a stream's

Bed load

Discuss the similarities between deltas and alluvial fans. Describe differences.

Both are structures that form as a moving body of water deposits most of the sediment load that it's carrying. The major difference is where that load gets deposited and what type of sediments comprise it. Alluvial fans are found at the base of mountain ranges, where high energy streams are reaching a plain. When no longer confined to a channel the energy level of the water drops, which makes the water unable to transport larger sediments, cobbles, gravel. If the water is no longer able to carry those sediments, they drop out and are deposited. Alluvial deposits are often poorly sorted. Deltas form where a moving body of water enters a non-moving body, most often a river reaching an ocean. Deltas are deposited in water and eventually build themselves up to be above sea level. By the time most major rivers reach the ocean they are carrying an immense sediment load, but that load is almost completely silt and clay, so delta deposits are usually very well sorted.

What are typical units of discharge?

Cubic feet per second

The total area drained by a stream and its tributaries is called the

Drainage basin

Hydraulic action, solution, and abrasion are all examples of stream

Erosion

A river's velocity is ____ on the outside of a meander curve compared to the inside.

Higher

Describe on way that incised meanders form.

Incised meanders may form as a result of a change in the base level of the stream. Uplift of an area may promote intense downcutting into the channel as base level is changed, thereby promoting the formation of an incised meander. Long-lived streams located high above base level may also formed incised meanders. In this case, uplifting of the area is not necessary in order to form the incised meander.

In what part of a large alluvial fan is the sediment the coarsest? Why?

Sediment is the coarsest near the head (upstream portion) of the fan because the largest sediment particles are mostly likely to be deposited near the fan head, where the streams' velocity is typically insufficient to transport the largest sediment particles.

How is the recurrence interval for a flood determined? How may new data affect the flood-frequency curve?

Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given precipitation event. The recurrence interval is based on the probability that the given event will be equaled or exceeded in any given year. For example, assume there is a 1 in 50 chance that 6.60 inches of rain will fall in a certain area in a 24-hour period during any given year. Thus, a rainfall total of 6.60 inches in a consecutive 24-hour period is said to have a 50-year recurrence interval. Likewise, using a frequency analysis (Interagency Advisory Committee on Water Data, 1982) there is a 1 in 100 chance that a streamflow of 15,000 cubic feet per second (ft3/s) will occur during any year at a certain streamflow-measurement site. Thus, a peak flow of 15,000 ft3/s at the site is said to have a 100-year recurrence interval. Rainfall recurrence intervals are based on both the magnitude and the duration of a rainfall event, whereas streamflow recurrence intervals are based solely on the magnitude of the annual peak flow.

Describe how stream terraces form.

Stream terraces first form as floodplains along the fringes of a stream channel, when water overtops the channel and spills out onto the floodplain. With rapid downcutting, tectonic uplift, or a change in climate, the stream may erode to a newer, lower level, creating a new floodplain and leaving the higher, older floodplain high and dry as a terrace.

What is base level?

The limit of downcutting. It is a theoretical limit for erosion of the Earth's surface. Downcutting will proceed until the streambed reaches base level. If the stream is well above base level, downcutting can be quite rapid; but as the stream approaches base level, the rate of downcutting slows down.

The average time between floods of a given size is

The recurrence interval

How does a stream widen its valley?

They are usually cut by streams. By removing rock and sediment from the stream channel, a stream deepens, widens, and lengthens its own valley.

Sandbars deposited on the inside of meander curves are called

Point bars