Reference no: EM13966935

Fluvial Landforms Lab

Stream drainage patterns often reveal useful information about the structureand lithology underlying surficial streams. A search for a distinctive drainage network on a topographic map or aerial photograph is a logical first step in the study of regional geography and geology. One of the most common drainage patterns is dendritic, which often develops on horizontally bedded sedimentary rocks or uniformly resistant crystalline rocks. Also possible are parallelpatterns, indicating moderate to steep slopes or parallel elongate landforms, and trellis, which commonly occurs atop dipping or folded sedimentary,volcanic, or low-grade metasedimentary rocks. Trellis patterns may also be structurally controlled in areas of parallel fractures. Similar to trellis but lacking the orderly, repetitive quality is the rectangulardrainage pattern. This pattern reflects jointing and/or faulting at right angles in the underlying bedrock. Radial patterns indicate volcanoes or domes of uniform lithology, while annularpatterns suggest volcanoes or structural domes that expose sedimentary rocks of variable resistance. Inversely, a centripetaldrainage pattern forms in structural basins, karst,or old lakebeds. Localized subsurface accommodation of drainage forms a multibasinaldrainage pattern (also called deranged). Deranged patterns may reflect hummocky glacial deposits, differentially scoured or deflated bedrock, karst, or permafrost regions. Review the textbook for specific guidelines in identifying drainage patterns.

Fluvial landforms are landforms shaped dominantly by the action of running water, whether in the form of overland flow or stream flow. Even in arid and semi-arid environments like Colorado, features produced by fluvial processes often dominate landscapes. Because vegetation density is low and soil mantles are relatively thinin arid regions, geomorphic and structural features are often very pronounced on topographic maps and aerial photographs.

The piedmontis a large, gently sloping surface that joins a mountain range to an adjacent basin or plain. A pedimentis a portion of the piedmont that consists of a sloping, relatively smooth bedrock surface that may be covered by a thin veneer of sediment. Pediments are thought to be erosional in origin. Alluvial fansare stream deposits that accumulate on piedmonts at the base of mountain fronts. They are fan-shaped in plan-view and convex-up in a cross-section taken parallel to the mountain front. Fans are commonly dissected by distributary channels or washes, all of which are probably not active at any one time. The coalescing of several adjacent alluvial fans along a mountain front formsBajadas. The transition from a series of single fans to abajada is complete when the cone shape of each individual fan is lost.

Bolsons are extensive, flat basins or depressions, almost or completely surrounded by mountains and from which drainage has no surface outlet. These arid region features characteristically have centripetal drainage patterns and commonly have level plains in their center called playas. Playa lakesmay form during heavy rainfall, but are usually short-lived, due to aridity and consequent infiltration and evaporation.

In areas of very different climates, some streams have a natural inclination to develop a sinuous or meandering channel pattern. The growth of a meander results from erosion of the outside bank of each bend and concurrent deposition of material on the inside of each curve. As the meander bend migrates laterally, the continued accretion of sediment on the inside of the meander curve forms a point bar.Meanders also migrate downstream because of more effective erosion occurring on the downstream side of the meander bend. Differential erosion causes these meander bends to become distorted and increasingly sinuous until a threshold of disequilibrium is reached. Once unstable, the river cuts across the meander loop to follow a more direct course. Abandonment of the old meander loop produces a crescent-shaped lake called an oxbow lake.

Periodically, a stream will form temporary cutoff channels during high discharge without permanently altering the channel pattern. This process, coupled with continued lateral migration of a meander and point bar deposition, forms a series of low ridges and shallow swales on the inside of meander bends called ridge and swale topographyor meander scroll topography.

Floodplains are formed by a number of processes, but lateral channel migration and vertical overbank deposition are believed to be the most important processes in operation. Most floodplains form by the interaction of a number of processes; the relative importance of these processes varies from situation to situation.

A natural leveeforms when a river leaves the confines of its channel and deposits coarse-grained material adjacent to the bank edge. This coarse-grained material continues to accumulate, due to subsequent flooding, and a natural levee is formed. On topographic maps only large levees are visible. These levees are recognizable as elongate, narrow ridges adjacent to a channel.

Occasionally, river flow breaches a levee, forming a gap in the levee called a crevasse. Sediment is deposited on the floodplain in a fan-shaped deposit of debris called a crevasse splay. These features tend to form on floodplains having well-developed levees which form barriers to surface drainage attempting to enter the river. These areas are called backswamps; they play an important role in reducing flood peaks downstream by trapping and temporarily storing floodwater, which has breached the levee.

Terracesare abandoned floodplains formed during a period when the river flowed at a topographically higher elevation. Terraces result when channel downcutting lowers a river's level, creating a new - lower and smaller - floodplain. Terrace surfaces represent relic features that are no longer directly related to current flood hydrology. Terraces form in response to changes in stream discharge or bedload sediment transport rates.

Use Google Earth to look up the following locations to help you answer the following questions. Once you ‘fly' to an area, zoom out and pan around to see the entire drainage basin patterns, or zoom in to see particular landforms.

Mount Rainier, Washington

1. (What drainage pattern is formed on Mount Rainier?

Lake Wales, Florida

2. What drainage pattern dominates this area?

Strasburg, Virginia

3. (a) What drainage pattern occurs in this region?

(b) What do the erosional patterns of the channels indicate about the relative resistance of theridges and valleys?

St. Paul, Arkansas

4. (a) What is the drainage pattern in this area?

(b) Qualitatively describe the drainage density in this area (i.e., does it look like it is relatively dense, or not very dense at all).

Bright Angel, Arizona

5. (a) What drainage pattern occurs on the Kaibab Plateau?

(b) What drainage pattern dominates in Bright Angel Canyon specifically?

Brandon, Vermont

7. (a) What is the fluvial landform represented by thewetlands along either side of Otter Creek? (Look for the green tree symbol to the left of Brandon, indicating the Brandon Swamp)

(b) What fluvial landform probably separates these wetlands from the river?

Ennis, Montana

8. (a) What type of channel pattern does the Madison River exhibit?

(b) The Cedar Creek Alluvial Fan represents alarge deposit of material transported from the local mountains. How could this alluvial fan be affecting the Madison River channel pattern?

Guadalupe Peak, Texas

9.(a) What geomorphic name would you apply to Salt Lake?

(b) What is the gently sloping area linking Salt Basin and Crow Flats to the Guadalupe Mountains?

Menan Buttes, Idaho

10.What is the fluvial landform represented by the crescent shaped sand mounds along the inside of the meander bends of the river (found to the left of Menan Butte)?

Coastal Processes Lab

INTRODUCTION

A beach is a dynamic environment in which the sediment that composes a beach is in constant motion. The movement is caused by waves and currents acting along the beach, and the amount of movement varies depending upon the number and size of waves that strike the beach and the speed and direction of the currents. Under stable conditions, the amount of sediment removed from a segment of the beach is balanced by the amount that is brought into that area, so that no net loss or gain occurs. If more sediment is brought in than is lost, the beach increases in area, i.e., accretion occurs. On the other hand, if more sediment is removed than is deposited, beach erosion occurs. Sediment losses increase during storms when wind and wave action are stronger than normal. However, the erosion that occurs during a storm is often balanced by deposition that occurs after the storm; the result being that the beach returns to its former state. Nonetheless, if you happen to have a home on a beach that erodes during a storm, then you lose your home! Erosion can also occur when vegetation is removed from a beach during development. Removal of vegetation allows both wind and water to transport more sediment than normal, so that net losses may occur from the devegetated area. In addition, erosion can occur when the supply of sediment to a coast is diminished, as for example, when dams are built across rivers leading to the sea. When this happens, sediment that was headed to the coast becomes trapped behind dams and sediment loss for the beaches exceeds sediment gain. Because development is occurring on most beaches, and because dams have been built across most rivers, the rate of sediment removal is increasing while the rate of sediment supply is decreasing. Consequently, most of the beaches in the United States and around the world are eroding.

BEACH EROSION RATES

The data table below lists shoreline changes that occurred along Sergeant Beach, TX between 1852 and 1988. There has been substantial coastal retreat since 1852. Use the data provided in this table to answer the following questions concerning beach erosion rates.

1. Determine the amount of retreat per interval (complete column C). Then add these values together and divide by the number of years of observation (as listed below):

a. Total amount of retreat = __________________

b. Number of years of record = _______________

c. Average amount of retreat per year = ______________ (round to the nearest tenth)

2. Calculate the average rate of retreat per year per interval and record those values in column E. To do this, first determine the number of years per interval (column D). Then divide the amount of retreat per interval by the number of years in the interval. (Example: 1852-1930 interval: 839 feet of retreat ÷ 78 years = 10.8 ft/yr)

a. The lowest average rate of retreat was ______________ feet/year

b. The highest average rate of retreat was ______________ feet/year

Rate of Retreat at Sergeant Beach, TX

     A               B                   C               D                       E

3. Calculate how far the beach will retreat (relative to the 1852 shoreline) by the year 2010 using the average rate of retreat you just calculated. Follow the steps given below.

a. Number of years in interval from 1988 to 2010: __________ years

b. Amount of retreat during the interval (number of years in interval multiplied by the average rate of retreat calculated in #1c.): _________feet

c. Add the value you obtained to the total amount of retreat during the 1852-1988 interval: Total amount of retreat = ______________feet

d. Repeat steps B and C using the lowest and highest average rates of retreat obtained in question 2:

i. Total amount of retreat based on lowest rate: ______________feet

ii. Total amount of retreat based on highest average rate: _________________ feet

e. What is the maximum difference in shoreline retreat estimates for the year 2010? _______________feet

4. The above estimates show a substantial variability of rates of shoreline retreat. What does this suggest about the reliability of estimates of future shoreline positions?

5. If it were your job to inform the people living in the Sergeant Beach area that their beach was eroding, which of the three estimates of retreat would you use and why? (Which do you think is the most accurate?)