Introduction
Stream drainage patterns often reveal useful information about the structure and 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 parallel patterns, 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 rectangular drainage 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 annular patterns suggest volcanoes or structural domes that expose sedimentary rocks of variable resistance. Inversely, a centripetal drainage pattern forms in structural basins, karst, or old lakebeds. Localized subsurface accommodation of drainage forms a multi-basinal drainage pattern (also called deranged). Deranged patterns may reflect hummocky glacial deposits, differentially scoured or deflated bedrock, karst (with sinkholes), or permafrost regions.
Review the textbook for diagrams on the main drainage patterns types.
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 thin in arid regions, geomorphic and structural features are often very pronounced on topographic maps and aerial photographs.
The piedmont is a large, gently sloping surface that joins a mountain range to an adjacent basin or plain. A pediment is 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 fans are 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 forms Bajadas. The transition from a series of single fans to a bajada 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 lakes may 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 topography or 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 levee forms 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.
Terraces are 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.
Directions
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 the ridges 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
6. (a) What is the fluvial landform represented by the wetlands 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
7. (a) What type of channel pattern (i.e., straight, braided, meandering) does the main channel of the Madison River exhibit? (not the small, secondary stream)
(b) The Cedar Creek Alluvial Fan represents a large deposit of material transported from the local mountains. How could this alluvial fan be affecting the Madison River channel pattern?
Guadalupe Peak, Texas
8. (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
9. 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)?