Unit 42


Unit Overview

This unit examines the various landforms and processes associated with stream erosion. The main sections are as follows:

  • Factors affecting stream degradation
  • Drainage patterns
  • Overcoming geologic structure
  • Regional geomorphology

Fluvial processes exert considerable influence on landscapes, and, in turn, the major factors influencing stream erosion are geologic structures, lithology, tectonic activity, and climate. Examples of geologic structures include hogbacks, cuestas, ridges and valleys, domes, faults, extrusive igneous structures, intrusive igneous structures, and metamorphic structures. The two variables related to lithology that affect erosion are slope and erosional resistance. Tectonic activity can greatly increase erosional rates, especially through regional "tilting."

Climate indirectly affects the evolution of a landscape; nevertheless, considerable landscape heterogeneity can exist within a single climate type. Drainage patterns often reveal much about the geology of a region, and an important characteristic of drainage pattern is drainage density. Erosional efficiency tends to increase with an increase in drainage density.

The five most common types of drainage patterns are dendritic, trellis, radial, annular, and rectangular. Stream drainage typically follows the underlying geologic structure. Streams can also flow counter to the geologic structure; these streams are referred to as superimposed and antecedent streams. Fluvial erosion plays a central role in several theories that attempt to explain regional geomorphology. The Davisian cycle of erosion holds that a landscape's underlying geologic structure is being acted upon by streams and other erosional processes, and thus the landscape is at a certain stage of degradation with the final stage characterized by a nearly flat surface known as peneplain. An alternative theory suggests that slope retreat reduces highlands to pediplanes, which are planes at the foot of mountains.

Unit Objectives

  • To outline the roles of geologic structure, lithology, tectonics, and climate in influencing fluvial erosion
  • To introduce terminology to characterize drainage networks and controls on fluvial erosion
  • To briefly consider how landscapes might change or evolve through time in response to river erosion

Glossary of Key Terms

Annular drainage A concentric stream pattern that drains the interior of an excavated geologic dome.
Antecedent stream A river exhibiting transverse drainage across a structural feature that would normally impede its flow because the river predates the structure and kept cutting downward as the structure was uplifted around it.
Butte Small, steep-sided, caprock-protected hill, usually found in dry environments; an erosional remnant of a plateau.
Cuesta A long ridge with a steep escarpment on one side and a gently dipping slope and rockbeds on the other.
Cycle of erosion The evolutionary cycle proposed by William Morris Davis that purportedly affects all landscapes.
Dendritic drainage A tree-limb-like stream pattern that is the most commonly observed; indicates surface of relatively uniform hardness or one of flat-lying sedimentary rocks.
Drainage density The total length of the stream channels that exist in a unit area of a drainage basin.
Geologic structure Refers to landscape features originally formed by geologic processes, which are sculpted by streams and other erosional agents into characteristic landforms.
Hogback A prominent steep-sided ridge whose rockbeds dip sharply.
Interfluve The ridge that separates two adjacent stream valleys.
Mesa Flat-topped, steep-sided upland capped by a resistant rock layer; normally found in dry environments.
Monadnock A prominent, not-yet-eroded remnant of an upland on a peneplain.
Pediplane A surface formed by the coalescence of numerous pediments after a long period of erosion has led to parallel slope retreat.
Peneplain The concept of a "near plane" developed by William Morris Davis to describe the nearly flat landscape formed by extensive erosion over long periods of time.
Radial drainage A stream pattern that emanates outward in many directions from a central mountain.
Rectangular drainage A stream pattern dominated by right-angle contacts between rivers and tributaries, but not as pronounced as in trellis drainage.
Stream piracy The capture of a segment of a stream by another river.
Superimposed stream A river exhibiting transverse drainage across a structural feature that would normally impede its flow because the feature was at some point buried beneath the surface on which the river developed; as the feature became exposed, the river kept cutting through it.
Trellis drainage A stream pattern that resembles a garden trellis; flows only in two orientations, more or less at right angles to each other; often develops on parallelfolded sedimentary rocks.
Water gap A pass in a ridge or mountain range through which a stream flows.

Unit Outline

  • Factors affecting stream degradation
    • Geologic structure
      • hogbacks are dikes that become steep-sided ridges (Fig. 42.1)
      • cuestas are less prominent ridges of sedimentary strata at a low-angle dip (Fig. 42.1)
      • parallel ridges and valleys produced by erosion of synclines and anticlines
      • domes are eroded to form circular cuestas (Fig. 42.2)
      • fault-line scarps are fault scarps that have been eroded (Fig. 42.3 & 42.4)
      • volcanic landforms eroded to plateau-like mesas or smaller buttes (Fig. 42.6)
      • metamorphic rock may show regional foliation
    • Bedrock type
      • rocks have varying hardness and tendencies to form vertical slopes
    • Tectonic activity
      • rivers are altered and even rejuvenated by slight tilting produced by shifting lithospheric plates
        • uplifting results in increased velocity and erosional capacity
    • Climate
      • landscapes in humid areas thought to be more rounded and eroded than arid regions, may be a misperception
      • must account for long-term climate change
      • climate does playa role in shaping the overall landscape in the long run
  • Drainage patterns (Fig. 42.11)
    • Drainage density is the erosional effectiveness of a drainage system
    • Radial drainage pattern shows drainage of conical mountains, often volcanic cones
    • Annular drainage occurs on domes, forms a concentric circular pattern
    • Trellis pattern often found on parallel folded or dipping rocks, flow in two directions
    • Rectangular drainage tends to occur in small areas with joint and/or faults
    • Dendritic drainage is a tree-limb pattern, found on batholiths or flat-rock areas; slopes toward trunk river
  • Overcoming geologic structure
    • Superimposed and antecedent streams (Figs. 42.12 & 42.13)
      • as streams erode downward across mountain peaks, a water gap is produced, the river becomes a superimposed stream
      • if a river predates a ridge that is pushed up tectonically, and is able to keep pace by erosion, it is called an antecedent stream
    • Stream capture (stream piracy)
      • one river captures a segment of another river
      • one river strengthened at the expense of another
  • Regional Geomorphology
    • Slopes and plains
      • cycle of erosion proposed by William Morris Davis
      • three elements of cycle are structure, geographic process, and time
      • high mountains eventually leveled to nearly flat peneplain
      • flat areas then transformed into mountains, highlands, hilly interfluves, and flat areas once again
      • most prominent not-yet-eroded remains called monadnocks
      • other geomorphologists proposed that flat areas reduced to pediplanes
      • humid areas may exhibit more convex slopes, with higher drainage densities than arid areas, which have more concave structures

Review Questions

  1. Describe the formation of a hogback and a cuesta.
  2. Explain how lithology (bedrock type) influences landscape evolution.
  3. List the five characteristic drainage patterns, using Fig. 42.11 as a guide.