GS104 Lab 9 Answer Key - Stream Table Experiments and River Systems

PRE-LAB READING QUESTIONS

1. Discharge - volume of water per unit time, flowing through a river channel (units are in cubic ft / sec)

2. Stream Gradient - slope of a river channel (rise / run), calulated by change in elevation along the channel divided by horizontal channel distance (units are commonly in ft/mi).

3. Saltation - a bedload transport process on river bottoms. Coarser sediment (sand through gravel) is transported by bouncing and hopping along the river bottom.

4. Base Level - the elevation of an "equilibrium surface", above which the river will tend to erode, and below which the river deposits. Sea level is the ultimate base level for a river.

5. Floodplain - low-lying area adjacent to a river channel that is periodically inundated with water during overbank discharge events (floodplain typically is inundated every 1 to 5 years)

6. Cut Bank - steep banks on the outside of a meander bend. The outside of the bend is subject to higher flow velocity and erosion, producing a cut bank as the river migrates laterally.

7. Point Bar - sediment deposit on the inside of a meander bend, resulting from decreased flow velocity and deposition.

8. List and define the three types of sediment load that may be transported by a river? bedload, suspended load, dissolved load

9. What surface process is the primary factor controlling the discharge of a river? Rainfall and amount of surface runoff.

1 STREAM FLOW (STREAM TABLE EXERCISE)

The highest velocity is on the outside of the meander bends, with the lowest on the inside. This results in cutbank erosion on the outside, and pointbar deposition on the inside.

EQUILIBRIUM

As the gradient of the stream table was increased, the flow velocity increased with the following observations:

(1) Meander loops were cut off (2) Channels were generally straightened, and eventually formed a braided stream network.

1a. The increase in slope of the stream table is analogous to increase in slope of the landscape. A good way to do this in nature is through tectonic uplift / tilting of the Earth's surface.

SEDIMENT TRANSPORT

Scenario - Increase in Discharge of Stream Result: more sediment is transported under higher discharge conditions. suspended load = chalk dust bedload = sand grains and granules Bedload transport style - rolling, sliding, difficult to see saltation.

2 STREAMS AND FLUVIAL LANDSCAPES

(1) Gradient = change in elevation per unit distance along the length of a stream channel.

(2) Method of Determining Stream Gradient

a. Gradient - the slope of a stream channel segment, calculated by change in elevation along the stream channel, divided by the horizontal distance along the channel length (commonly measured in ft / mi).

i. Method for Calculating Stream Gradient on a Topographic Map

Step 1 - find two points along a stream channel, where contour lines cross (V- shape contours, with tip of V pointing upstream

Step 2 -determine the contour interval on the map

Step 3 - count the number of contour lines between your two points in Step 1, and determine the elevation difference between the two points on the stream (determine the elevation difference in feet)

Step 4 - measure the total horizontal map distance between the two points in Step 1. Use a sheet of paper, making tic marks, twisting the paper along the stream channel, breaking the stream channel into short straight-line segments. YOU MUST MEASURE THE HORIZONTAL MAP DISTANCE ALONG THE LENGTH OF THE STREAM CHANNEL. Use the graphical scale of the map and determine the length in miles.

Step 5 - Calculate stream gradient = vertical elevation difference / horiztontal distance

Example Gradient Problem: You find two contour lines that cross a stream channel on a contour map. It is determined that the vertical elevation difference between the two points is 40 ft. You then measure the horizontal map distance along the stream channel and determine a distance of 100 miles. The stream gradient is calculated as follows:

Gradient = change in elevation / horizontal distance = 40 ft / 100 miles = 0.40 ft / mi

(3) Gradient of North Fork of the Middle Fork of Willamette River (Chucksney Mtn, quad)

Gradient = 320 ft / 4.8 mi = 66 ft/mi

(4) Gradient of Willamette River (Salem, Quad)

Gradient = 25 ft / 14.6 mi = 1.7 ft/mi

3 (5) Gradient of the Columbia River (Sauvie Island, quad)

Gradient = 10 ft / 100 mi = 0.10 ft /mi

(6) Stream gradient decreases from the headwaters of a river to the mouth of the river. As the river approaches sea level, the gradient becomes very low. The lower the gradient, the lower the stream flow velocity (all other factors being equal). Lower gradient rivers are associated with less erosive power, and more depositional storage. Higher gradient rivers are associated with more erosive power, and increased erosion.

MONMOUTH QUADRANGLE EXERCISE

(1) The feature on the Willamette east of Independence is a meander loop, with migrating point bar on the inside of the bend (across the river from Independence), and cutbank erosion on the outside of the bend (on the Independence side). Meander scrolls are evident in the pointbar topography across the river from Independence.

(2) Yes, the river is migrating towards Independence, with erosion to the outside of the bend. Yes Independence should be worried, and should employ some erosion control measures (e.g. rip- rap).

(3) A meander loop is about to be cut off in the vicinity of Helmick State Park. An oxbow lake may form in the future, as erosion progressives.

(4) WOU is located on a low-terrace of the Willamette, elevation of the Natural Sciences Building is ~210 ft above sea level.

(5) See cross-section below:

4