Introduction
The volume of water on Earth is estimated to be about 1.36 billion cubic kilometers with- 97% in the oceans
- 2% in glaciers
- l% in streams, lakes, ground water, and the atmosphere
Streams have played an important role in history. Think of the major cities which were located close to a river for ease of transportation. During times of flood (the 1993 Des Moines, Iowa flood, for example) many residents probably wished that they had not settled so close to a river.
Water is continually recycled from the oceans, through the atmosphere, to the continents and back to the oceans. This cycle is powered by solar radiation and is effective because water changes phase (solid to liquid to gas) easily at surface conditions. Water evaporates from the surface waters which are heated by solar radiation and the water vapor rises into the atmosphere. The warmer the temperature of the atmosphere the more water vapor can be held in the atmosphere. As the atmosphere cools, condensation can occur with the formation of clouds or precipitation. Most of the precipitation falls directly in the ocean and the cycle begins again.
On land the process is more complicated. Some of the water falls as precipitation in the form of snow or water. Some is locked up in glaciers, some percolates into the surficial material and becomes part of the ground water system, and some flows in channels.
When water evaporates energy is needed to drive the reaction. When water condenses energy is released.
Running Water
Most flow is turbulent; flow lines cross and mixing between layers takes place. The opposite is laminar which takes place at very slow flow rates and is enhanced when the density and viscosity are low.
The gradient is the slope over which the stream flows and is measured in feet (or meters) per mile. The gradient changes over the course of the stream; typically it is steep near the head waters and flattens out near the base level. The ultimate base level is sea level - streams will not erode their channels below their base level. Many temporary base levels can exist along a stream's path. Erection of a dam results in creation of a lake which serves as the temporary base level for the up stream part of the river.
The discharge of a stream is the product of the velocity (feet/second) times depth times height. Depth times height gives the cross sectional area. The shape of the channel is important because of the reduction in velocity due to friction along the channel margins. In a natural situation, an increase in discharge (cubic feet/second) will result in an increase in all three parameters - the river becomes wider, deeper and flows at a higher velocity.
The velocity of a stream usually increases downstream. The gradient typically decreases but gravity, elimination of bed roughness and an increase in water from the entry of tributaries tends to increase the velocity.
Transportation and Deposition
Streams carry sediment as part of the bed load (along the stream bed), as part of the suspended load and as part of the chemical load. The greater the energy of the stream the larger the size of material that can be carried (the competency of the stream) and the larger the total suspended load - capacity . As the velocity of the stream increases material in the bed load is moved into the suspended load. Separation into a bed load and a suspended load assists in sorting the sediment; the collection of similar size material. If material in the bed load is lithified it is effectively removed from the transport system. For chemical sedimentary rocks, deposition is a function of the chemistry of the solutions. When the concentration of the appropriate ions exceeds their solubility, precipitation will occur.
Environments of Deposition
Reconstruction of the environment of deposition is a goal of those geologists that study sedimentary rocks. A "cartoon" depicting Environments of Deposition will help place common environments in perspective.
- Continental - Terrestrial
- alluvial fans - near the base of high elevations. An alluvial fan forms at the base of steep mountainous regions. As there is relatively little energy available to move the material about, alluvial fan sediments are poorly sorted (wide range of sizes).
- fluvial environments - river systems. Sometimes the drainage pattern tells you quite a bit about the geology of an area.
- Braided streams develop when the channel is not well defined and when the stream receives a great deal of coarse sediment. Braided streams typically transport sediment across alluvial fans
- Meandering streams develop when the channel is well defined. The cross-section of a meandering bend is asymmetrical; steepest on the outside (where erosion is taking place) and a gentle slope on the inside (where sediment is being deposited - point bars). With time the meanders migrate laterally across the floodplain. During an increase in discharge the stream may cut across the narrow neck of land separating adjacent meanders. This isolates the bend from the stream flow and produces an ox-bow lake which will eventually fill with sediment.
In the animation above the river is eroding the left-hand side of the bank and you are seeing a cross-section of the channel. Note the growth of a point bar on the right-hand side of the bank. You are also seeing a map view of the growth of the meandering stream. Note what happens when a cut-off occurs.
Another perspective on the evolution of a meandering river is given in this animation. What is the name given to the abandoned meander?
- If the temporary base level is lowered a stream will attempt to adjust to lower its valley. In this photograph of the Snake River in Idaho you can see two terraces. The highest terrace is the oldest. Note that the ages of these terraces does not conform to the principle of superposition! Think about the history of this area as reflected by these features.
- Lacustrine - Lake Environments. Lakes are rather ephemeral features. Sedimentation acts to fill up the lake and the area may become swampy and then dry land over time.
- Aeolian - Wind Deposits. Check out the images of Dune Fields and visit Great Sand Dunes National Monument in Colorado
The "preservation potential" of sediment deposited in the continental-terrestrial region is quite low. Shifting of river systems, advance and retreat of glaciers, the action of the wind, etc. may erode older sediment/sedimentary rocks and remove the material.
- Transition Zone
The preservation potential of material deposited in the transition zone is high. Much of the sedimentary rock preserved accumulated in the transition zone. Why? Keep in mind, however, that if the transition zone is along an active continental margin (a subduction zone, for example) that accumulated material will be effected.
- Marine
- Submarine Fans: Continentally derived sediment transported by turbidity currents tend to be rather poorly sorted due to the absence of pervasive currents.
- The Pelagic Component: Wind transported debris (often volcanic in origin) plus shells of the life forms that live in the water column.
Think about the nature of material that can accumulate in the marine environment. Thick sections of shales and/or mudstones (the pelagic component) interrupted by sandstones with evidence of a terrestrial origin (the submarine fans).
In the deep ocean basins calcite (calcium carbonate) is not stable and dissolves. Thus, below the CCCD (calcium carbonate compensation depth - ~ 10,000 feet) there is no carbonate in the sediments. These deep ocean sediments are made up primarily of silica (quartz, chert and amorphous silica) and clay minerals.
The preservation potential of material deposited in the marine zone is also high but thick accumulations are rare given the source of sediment.
The subject of facies was introduced in the chapter on Sedimentary Rocks. This would be a good time to review it. Sedimentary facies can be defined on different criteria. In the examples that follow grain size and grain type will be used to recognize three facies:
- Terrestrial - Sand
- Near Off Shore - Muds
- Off Shore - Carbonate
In this case the shoreline stays constant over time and the boundaries between the facies are vertical.
If sea level were to rise or the level of the land were to subside, the shore line would move onto the continental area and with time the facies boundaries would track the change in relative sea level. This is a Transgression
With time the mud facies will overlie the sand facies and the carbonate facies will overlie the mud facies. Note that a vertical section (carbonate over mud over sand) records the lateral variations that existed at one instant in time.
If sea level were to fall or the level of the land were to rise, the shore line would move away from the continental area and with time the facies boundaries would track this change. This is a Regression
