Geology 111/18
Drainage Basins
Bedrock Control of Stream Patterns
Sedimentary Basins
Alluvial Fans
Alluvial Plains
Meandering Streams
Braided Streams
Deltas
I. Drainage Basins. The erosional part of a river system; the entire area drained by a river and its tributaries. Typically pear-shaped in map view with the main river emerging from the narrow end, which often marks the mountain front or edge of an uplifted region. The mountain front is defined here as the boundary between net erosion and net deposition.
A. Stream Patterns: The geometric relationship of tributaries and trunk rivers as they look in map view. The pattern is largely determined by the nature of the underlying rocks of the drainage basin.
1. Dendritic. Pattern of branching tributaries, resembling branching habit of an oak.
2. Rectangular. Tributaries oriented at more or less right angles to one another. Usually indicates the present of orthogonal joint (fracture) sets.
3. Trellis. Short-headed tributaries flow into main trunk streams. Usually indicates structural control, which consists of long linear bands of resistant and non-resistant rocks resulting from folding.
4. Radial. Streams radiate outward in all directions; common on volcanoes.
II. Sedimentary Basin. The depositional part of the river system (and beyond). Grain size and morphology of the depositional system tends to change with distance from the mountain front, reflecting increased transport distance and decreasing slope, respectively.
A. Alluvial fan. Apron of sediment deposited at mountain
front where drainage system abruptly changes its slope; consists of coarse
gravel and sand. Fan-shaped,
hence the name. Each fan is
tied to its own drainage basin. Adjacent fans may coalesce if there are several
drainages exiting the range to form a bajada. The East Mesa along the
B. Alluvial plain. Low gradient surface that extends from the toe of the alluvial fans to the shoreline, across which rivers flow. Alluvial: pertaining to or composed of alluvium, a general term for material deposited by a stream.
1. River types are determined by slope and caliber of transport load, defined from their channel appearance in map view.
a. Braided rivers have straight courses marked by divergence of channels around bars during periods of low flow. They have relatively high gradients or slopes and transport sand and gravel by entraining it along the bed.
b. Meandering rivers have channels that consist of sinuous loops (meanders). Erosion takes place on the outside of the loops (cutbanks), and deposition takes place on the inside of the loops (point bars). As a result, the loops migrate with time. Sometimes they get cut off from the main channel to form oxbow lakes. Meandering rivers have low gradients or slopes and transport sand and abundant mud, the latter in suspension. Other features include natural levees, elevated stream banks where sandy sediment is deposited during floods, and floodplains, the nearly flat regions adjacent to the channel that receive flood water. Floodplains are made of deposits of silt and mud (good farmland, poor places for communities).
C. Delta. Platform
of sediment deposited at river mouth where it enters standingwater, such as
lakes, reservoirs or the ocean.
Flow velocity decreases abruptly, causing wholesale deposition of
fine-grained sediment. Named by Herodotus from the
1. Distributaries form where the gradient of the river system decreases, causing the channel to split into numerous subchannels; results in fan-shaped geometry of delta.
2. The ideal delta consists of three distinct sets of internal beds: bottomset beds formed of mud carried beyond the delta proper and deposited more or less horizontally in the basin; foreset beds formed of sediment deposited at an angle on the sloping front of the delta; topset beds formed on the top of the delta by deposition in the distributaries or between them during floods.