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Terrestrial sedimentary environments II - streams


San Pedro River, AZ
Streams are the primary means of transporting sediment across the continents. Almost all terrestrial depositional systems have streams and their associated facies, even arid or hyper-arid settings. Streams are also the primary agents of erosion and denudation on the continents. They represent hazards in flood plains and are a chief source of water. Last, they are easy to access and guage. As such, they are heavily studied and reasonably well understood compared to many other depositional systems.

Recognition:

In the modern world, streams are sinuous, channel-forming elements. They commonly leave course-grained deposits, even in sandy and muddy systems. However, they are associated with a broad range of physical, chemical, and biological processes. They have relatively gentle grades where they are depositing sediments ranging from 0.5 deg.- 0.01 deg. They can be relatively short (~1 km) to thousands of kilometers long. Commonly, they are 1-2 m deep, though the largest streams can be more than 15 m deep.

Before we can deal with the sedimentology of fluvial environments, there are some preliminaries:

Stream properties:


Flow stages: We speak of the following general flow stages:


Maturity:

Remember that when a stream is new, tectonic movement of the ground over which it flows has recently occurred, or its base level has recently changed, a stream will actively erode or aggrade in order to reach its graded longitudinal profile. This happens by two means:

These may occur in:

Braided stream channels are broad and shallow, often about 1 m deep. As such, one finds:

Bars

Primary braided stream deposits are the remains of bars separating channels. These can be:

Bars are usually ephemeral, but can be stabilized if colonized by vegetation.


Brahmaputra River from space from Indianexpress.com
Historical note: Braided streams are usually small, but can be very large:


Jim Jim Falls, Northern Territory, Australia
Braided streams in the rock record: How do we distinguish braided from meandering stream deposits. Both are: but beyond that, braided streams:


Columbia River, BC from Rhine-Meuse Delta Studies
Anastamosing streams:

Specs:

Anastomosing streams are not very common. Their map profiles resembles those of braided streams, except that in place of unstable bedforms separating channels, there is stabilized floodplain. Typically occur in regions of intense deposition, including:



Okavango "Delta"
Terminal splay:

A typical setting for an anastamosing system is in a stream that is rapidly losing flow into the substrate or due to evaporation (arid and semi-arid climates are typical). The flow depth decreases and velocity decreases, leading to deposition of broad sand sheets in or near thin, shallow channels. These channels will avulse rapidly and repeatedly over short time and length scales creating a complex distributary network. These are called inland deltas or terminal splays. A first class example today is the Okavango Delta in Botswana (rt.).

Straight streams:

These commonly occur in three settings: