Geography 3421 - Geomorphology - Fall 2003 Chapter Outline for Easterbrook's Text
Chapter 6 - Fluvial Landforms

Drainage Systems - Playfair's law states basically that streams produce the valleys that they flow in.


Origin of Stream Courses: The factors that govern the course of a stream include the initial slope of the land, random headward erosion, and selective headward erosion:

• Consequent Streams: are those that run downslope over a newly exposed land surface.
• Insequent Streams - result from random headward erosion, thus have no particular preferred or guided direction with respect to other streams.
• Subsequent Streams - result from selective headward erosion, following outcrops of less-resistant rock, fault zones, joints, etc.
• Obsequent and Resequent Streams - obsequent streams flow in the opposite direction to the subsequent streams of an area, whereas resequuent streams flow in the same direction as the consequent streams.

Drainage Patterns

• Dendritic Patterns - are random, like tree-branch patterns, and reflect homogeneity or lack of preferred directional features in the bedrock.
• Trellis Patterns - sets of parallel or nearly parallel streams that flow into "trunk" streams at near-perpendicular angle.
• Rectangular and Angular Patterns - "blocky" or "squarish" stream pattern as though streams flow in gaps or lines of weakness such as joints or faults, between blocks of bedrock.
• Radial Patterns - characterize drainage off of domes or volcanic cones.
• Annular Patterns - circular-maze pattern found on domes developed in layered rocks of variable resistance.
• Centripetal Patterns - streams flow into a closed basin ("closed" or "interior" drainage)
• Parallel Patterns (rare) - streams flow parallel due to strong control by a set of joints, system of parallel faults, or straight outcrops of resistant beds.

Stream Capture - can occur when the boundaries or divides between drainage systems migrate due to differences in rates of headward erosion by streams. 

• Abstraction = stream capture that results from undercutting of a stream by a more vigorous one.
• Intercision = stream capture as a result of meandering of one stream into the channel of the other.

Floodplains - are the area or part of a stream's valley that the stream has actually been meandering or braiding over, or that can be inundated by a flood.  Depending on whether the stream is aggrading or degrading, the floodplain may be built up of alluvial deposits, or cut into bedrock with only a thin covering of alluvial material.

• Channel deposits - are the material in the actual channel plus whatever is left after the channel migrates as by meandering.
• Overbank deposits are deposited from floodwaters that spread away from the channel

Pediments - are graded erosional surfaces typically found sloping away from desert mountain areas.  With time the pediment works upslope as the mountains wear back to form inselbergs ("island-rocks").  The relative importance of stream erosion / lateral planation versus sheetwash / rillwash have been debated for mare than 100 years.


Alluvial Fans - are cone-shaped accumulations of alluvium formed where a stream emerges from a mountain valley onto a flatland.  They are most notable in arid regions where they border fault-block mountains, but can be found (typically in the same geologic setting) in humid areas as well.  In any case they appear to grow sporadically, as during flash floods when there is enough water to carry sediment flow across the fan surface.  Alluvial fans can develop so as to merge almost imperceptibly with pediments upslope, or with rock fans, which are an intermediate form.


Deltas - are built from sediment deposited where a stream empties into a body of standing water.  Many types can be recognized, depending on the balance between fluvial factors and marine factors such as waves and tides.  The Mississippi River delta is a "bird-foot" type which grows with comparatively little reworking by tides or waves (other than occasional storms).  The Nile and Rio Grande deltas are worked over by waves.


The Cycle of Erosion - Powell's concept of base level (the lowest elevation to which a stream can erode) leads to the idea that the end stage of stream erosion in an area should be a nearly level surface where no stream has enough gradient or energy to erode any deeper.  Davis's ideal erosion cycle starts with a flat or nearly-flat surface (peneplain) that could be the end result of a previous cycle, or perhaps something such as a newly uplifted stretch of seafloor, and then eventually is worn down to near-flatness again.

• Sequence of Forms in the Cycle of Erosion:  Davis used the terms "youth," "maturity" and "old age" for a three-fold division of the stages or succession of land forms in his ideal cycle.  Though useful as a descriptive concept or learning tool, there are problems with applying these terms in any more than a very general way.
  • Youthful landscapes are characterized by rapidly downcutting streams that produce V-shaped valleys, with flat divides between streams, where the original flat surface is yet to be significantly attacked.
  • Mature landscapes have streams that have made progress toward base level, and so begun to erode laterally as well as downward.  The valleys develop flat floors or floodplains that are still narrow enough for the meanders to reach entirely across.  Floodplains make up most of the flat ground in the region as the flat-topped divides and all other ground are in slope; topographic relief is high or at maximum.
  • Old-age landscapes have streams that mainly erode laterally as they are essentially at base level.  Most of the landscape is, or has been, floodplain and the active meander belt occupies only part of it.  A few isolated hills (monadnocks) may be the only features that have escaped erosional flattening.
• Interruptions to the Cycle of Erosion - Regional uplift is like a fountain of youth for the stream cycle, as the term "rejuvenation" that is used for it implies.  Uplift increases gradients and stimulates downcutting by streams, producing terraces, entrenched meanders.  No name is officially used for the effects of regional subsidence, although some of its effects could perhaps be seen as premature aging.

Cyclic Stream Terraces and Erosional Surfaces - result from episodes of renewed downcutting that form new valleys nested within pre-existing floodplains.  Many are manifestations of oscillations of crust elevation or sea level fluctuations, but other factors could include climate changes, upstream changes in drainage basin, and so on.

• Types of Cyclic Terraces
  • Cut-in-Bedrock Terraces - as the name implies, the stream is working on bedrock and undergoing periodic rejuvenation.
  • Fill Terraces - are created on alluvial material that the stream previously deposited during an episode of aggradation.
  • Cut-in-Fill Terraces - are basically fill terraces that are being incised to make new floodplains at lower levels.
  • Nested Fill Terraces - multiple downcutting/rejuvenation cycles intermixed with episodes of aggradation.
• Correlation of Cyclic Erosional Terraces - it may be possible to reconstruct a stream's former profile by correlating the remnants of its terraces.  The result would be expected to resemble the stream's current profile, but at a higher elevation.

Noncyclic Surfaces

• Stripped Surfaces - result when/where erosion encounters a resistant bed below easily eroded beds.  The landscape takes on the shape of the resistant layer: a basically flat surface or plateau if the bed is horizontal, undulating if it is folded, and so on.  Once most of the hard layer has been removed, the landscape should revert to rougher topography.
• Marine Surfaces - shaped underwater by waves, which lose erosional power with depth ("wave base" is half the wavelength).  Nearshore deposition can aid the levelling process.  Uplifted marine terraces frequently have a stairstep appearance on uplifted coastlines, and can also be identified by remnants of marine deposits on their surfaces.

Last revised x/x/x 2003 by MAJordan