Geology - Fluvial Systems
Posted On: 2020-08-15
Geology 101 - Gale Martin - Class Notes
Fluvial Systems, or rivers, are by far the most abundant and important erosional agent on the surface of the earth. The information covered here is but a small portion of that needed to understand the dynamics of fluvial systems. One can approach rivers as components of evolving landscapes, means of sediment transport/deposition and agents of destruction (via floods). They are constantly changing. Rivers are one of the few agents of erosion that man can actually notice the forces at work during his or her life span. Yet few people are aware of the longer, geologic life spans of rivers and how man interferes (or at least tries to interfere!) with the shape of this "living", dynamic force.
Fluvial systems are produced by the collection of surface runoff in topographic lows. The water is under the force of gravity; draining from higher elevations where precipitation has occurred (rain, snow, sleet, etc.) toward lower elevations where it collects in BIG puddles (lakes, oceans), etc.). These "lows" are referred to as channels when they become modified by the erosive nature of the flowing water. The moving water has many names (rivers, streams, rills, rios, creeks, etc.) but is known to a geologist as a fluvial system. (Let's use the terms "river" or "stream" for this class.)
The head of a river consists of a network of smaller streams, tributaries, the at drain a large area. The collection area is referred to as its drainage basin and consists of all the area between any topographic "highs", or drainage divides, that surround and separate it from other river systems. As it drains the land it collects water and sediment; carving and modifying the surface. When it reaches the ocean, it releases the water through a system of distributaries, which branch out and form the river's mouth.
The water in a river is seeking the lowest possible level to which it can flow (base level). As it winds over its course, a river flows for many miles and attempts to reach an equilibrium with the surrounding environment (graded stream). Many changes occur: water is contributed through precipitation and tributaries, rock outcrops get in it's way, and low lying basins temporarily slow it down. But every river eventually flows to the lowest reaches on the surface of the earth: the oceans, or ultimate base level.
Factors that Influence Erosion and Deposition
As the water flows over miles of rock and soil it greatly alters the shape of the surrounding land. Moving water has energy and is able to do work. (Mankind will often use it to drive turbines that generate hydroelectric power.) This energy is used by the river to move sediment. The faster the river flows - the more erosional its nature. Rivers can erode or deposited tons of sediment along it's course depending on several factors.
The slope of a river channel greatly affects it's flow. A stream's gradient, or slope, is a measure of the drop in elevation that a river has over the distance that it travels. Rivers in mountainous regions usually have high gradients and fast flows. Sediment in these regions is quickly removed and transported to areas of lower gradients, such as flat lying basins.
The stream's velocity is a measurement of rate at which the water flows. Though rivers with steep gradient have high velocities, the velocity in any given river varies depending on the river path, the shape of the channel and roughness of the rock that forms the river bed. Friction in a river channel slows the flow and causes irregular patterns or turbulent flow. In a meander, the velocity of the stream will be concentrated on the outside of the curve, where the water is deflected by the bank. Any increase in velocity within a stream will result in erosion.
The volume of water that a stream carries can greatly influence its erosional behavior. For most rivers, the volume of water carried will increase from it's head to the river's mouth. This is due to the number of tributaries along its course. The discharge of a river, or the volume of the stream at a given time, is affected by the amount of precipitation that occurs. During spring rains/winter melt, a river must carry a greater volume of water downstream in a short span of time. This increase in discharge results in flooding, or overbank discharge. Within an individual river system, an increase in discharge results in erosion.
The stream load also is a factor that determines if erosion occurs. The stream has a limit to the amount of sediment it can carry (capacity) and the size of particles that it can move (competence). These limits are dependent on stream discharge and velocity. Generally speaking, the greater the flow of a stream, the higher the competence and the greater the capacity. If an "outside force" disturbs the river's equilibrium, erosion or deposition will occur. (Mankind has a tendency to do this quite often. Building a dam to block a river results in changes that induce deposition behind the dam and erosion at the spill gates.)
Erosion in a stream channel results in extension of the river system throughout its drainage basin. A river can erode channels by lifting and carrying sediment, abrading the river banks or dissolving any soluble rock (see Denudation). Erosion of the stream bed resulting in downcutting and deepening of the channel. This steepens the sides and results in unstable slopes. Mass movement of debris into the channel brings the slopes back to the angle of repose and produces a "V" shaped valley configuration. Along meandering streams, the outside curves, or cutbanks, become undercut by the force of the stream's velocity and lateral erosion occurs. The valley widens as the meander erodes and extends itself. A stream increases its length by headward erosion on the uphill end of its tributaries. Here the valley grows longer with additions of small gullies and rills which mark the outer reaches of its drainage basin.
Not all sediment sizes are eroded at the same flow or stream velocity. Obviously, heavier particles require greater velocities. But clay size particles are also harder to erode. Clay particles are flat and closer to the stream bed and may be "sticky" in character. Higher energy is needed to pick up a clay grain than those needed for silts and sands.
Once a grain has been eroded, a stream can carry it in a variety of ways. The bed load consists of larger sediment grains. Most gravels and pebbles are too heavy to be carried all the time. They move down stream by traction, i.e., rolling and sliding along the bed. Sand particles typically move through a process called saltation. They bounce and leap along the bottom, ricocheting and hitting other grains on the river bed. The suspended load usually consists of clays and silt size particles. These grains are light enough to move in the water column. Rivers which flow through regions containing weathered silicates are typically "muddy" looking due to the suspended clays in the water. Most rivers also carry a dissolved load. This consists of the ions picked up during chemical weathering (dissolution) of rocks in the drainage basin. Rivers that flow through carbonate dominated terrain (Florida, Bahamas, Aruba, etc.) contain only dissolved loads and appear clean and clear.
Deposition of sediment occurs whenever the river looses enough energy that it can no longer carry all of it's load. Any decrease in stream flow will result in deposition. Examples include: --where a mountain stream enters a flat basin area the drop in gradient results in a decreased stream flow; --a sudden decrease in velocity occurs when a stream enters the ocean or any standing body of water; and --sediment is deposited as a river's discharge drops with receding floodwaters. Heavier materials, silts and sands, are the first to be deposited. Clays can remain suspended for long periods of time even after the stream's velocity has stopped.
(See your text for pictures/figures. Figures remain the best method of landforms affected by rivers.) Rivers produce different erosional and depositional features based on climate and tectonics of the region. Let's generalize and group them into similar styles.
Mountain streams are usually erosional by nature due to high gradients and fast flows. Sediment is quickly removed and carried downstream. Waterfalls and steep V-shaped valleys predominate. Resistant layers result in differential erosion forming ridges and overhangs that water cascades over. The pounding falls (hydraulic action) results in undercutting at it's base. The overhang eventually collapses and a steep set of rapids remains. Potholes are commonly formed as rocks swirl and grind against the bare bedrock in the stream.
As a stream leaves steep mountainous regions a drastic drop in gradient results in deposition of sediment. In arid environments, alluvial fans are produced at the base of the mountain as streams shift to avoid the accumulating pile of sediment. Given sufficient time neighboring fans coalesce to form bajadas. The streams that exit the mountains commonly become intermittent in nature and often percolate into the porous gravels. The dissolved load may even be precipitated along the base of the fan complex. Enclosed basins may contain playa lakes; dry and alkaline during most of the year, they fill during flash floods and rain storms.
In regions were water is more abundant, a stream that exits mountainous regions must still contend with excess sediment loads. The coarse material is deposited in sand bars that quickly fill the channel. The stream spreads out and produces a flat broad valley with shifting bars and diverting streams. This braided stream complex is common around mountain ranges with plentiful glacial melt or high spring run off.
After a river settles into a main channel complex it typically begins to meander and produce features common to flood plains. The discharge of a river varies greatly between seasons and over the years. It typically overflows its banks and deposits sediment along its sides to produce a flat broad area called it's flood plain. As the river overflows a large pile of coarser sediment, known as a levee, accumulates next to the main channel. The levee increases the depth of the channel and can alleviate minor floods. If the flood plain remains flooded for major portions of the year, vegetation suitable to watery environments begins to grow and a swamp can develop.
The main channel of a river is not a permanent feature. Sand bars are commonly deposited throughout the channel and the stream's flow shifts within the flood plain through erosion of cutbanks and deposition of point bars. Through time meanders become elongate and exaggerated. With an increase in stream flow a meander can develop a cutoff. This shortens the route the river must take to traverse its course. Sand bars eventually isolate the meander and produce an oxbow lake. As sediment fills the oxbow, a meander scar develops. (Flood plains, produced by lateral erosion and deposition (due to flooding) are broad, flat fertile areas which have attracted farmers for thousands of years. It's still a part of a dynamic system -- flooding is part of a river's cycle. Mankind can't stop it.)
Whenever a river enters a standing body of water, be it a lake or an ocean, the velocity of the water drops quickly. Deposition of sediment blocks the river's mouth and it divides into a complex set of distributaries. A large wedge of sediment called a delta, eventually forms at the river mouth. The delta's shape is determined by many factors. The sediment input from the river is often reworked by tidal forces, waves and longshore currents. The delta can be stabilized by growth of vegetation in the form of swamps and marshes.
The appearance of a river and the ensuing landforms depend on numerous complex and interacting factors. Tectonics (uplift or mountain building events) and climate changes can readily alter the pattern of erosion/deposition and modify it's drainage pattern. Most of the landforms and surface topography of the world are developed by the dynamic nature of running water both from the present and the recent geologic past.