Steams Dynamic Equilibrium
(This module was developed by Dan Mecklenberg, Ecological Engineer with the Ohio Department of Natural Resources Division of Soil and Water Conservation and Jessica D'Ambrosio, Program Coordinator in the Department of Food, Agricultural, and Biological Engineering at The Ohio State University.)
Introduction:
This module was developed to help introduce you to various stream processes and how they are related to each other. This module should also help you think about what would cause a stream to be out of balance. You will use the Lane Balance as an example of one commonly used tool for understanding how stream stability is affected by actions we take on the landscape.
Understanding the Stream System
Streams and rivers need room to increase in size during large storm events and to change their shape and location in response to those storm events. Understanding concepts of fluvial geomorphology -- or the study of how flowing water shapes land -- can aid us in understanding how streams systems respond to both natural and human-caused influences. Flowing water can change the landscape through a combination of factors that include:
- Gravity, or the slope of the stream banks.
- Friction, which is a function of vegetation on the bed and banks, the type and size of sediments that make up the bed and banks, the slope of the streambed, and how many bends the stream has.
- Velocity, the speed of the water flowing through the stream channel.
- Quantity, the volume of water and sediment moving through the stream channel.
Like any physical system, streams tend toward balance -- or equilibrium -- between all of the above factors. The quantity and movement of water and sediment are the primary influences on the equilibrium of a stream system. In stable streams, the balance between these factors usually occurs within the confines of the stream channel. The stream adjusts its size and shape to transport, as efficiently as possible, water and sediment from areas of higher elevations to areas of lower elevations to reduce the energy associated with flowing water -- or stream power.
Stream Discharge
The flow -- or discharge -- at which moving sediment forms or removes bars, forms or changes bends, and generally shapes the channel is called the bankfull discharge. It is commonly thought of as the flow that fills the channel up to the top of banks, just before it spills out of its banks. Based on the results of many research studies across the nation, bankfull discharge occurs less than a handful of times each year and corresponds to the 1-2 year return period storm event (a single rainfall event that is likely to occur only once every year or two years). In Ohios streams, bankfull discharge may be associated with a return period that is less than the 1-year storm event. That is, the size storm that will cause streams to fill up to the tops of their banks often occur more than once a year. Each bankfull discharge rain event does not transport much sediment, but because bankfull discharges occur multiple times each year, when added together, they are the most effective at transporting sediments through a stream system.
Slope
Bends in the stream channel are called meanders (Figure 1). A meander is formed because of something called velocity distribution, which means the speed of flow will vary as it moves from one bank to another at any point in a stream. For example, water flows faster on the outside edge of any small curve and slower along the inside edge because water near the outer bank has to travel further (Figure 2). The fastest moving flow of water is called the thalweg and is shown in Figure 2 as the black dotted line. Think of the thalweg as a roller coaster that speeds up in riffles, slows down in pools, moves from side to side and up and down with depth, and rotates counter-clockwise as it moves around bends. Sediments deposit on the inner edge of a bend because the water, moving slowly, does not have enough energy to carry its sediment load, and fine sediments drop out of the water column. The faster moving current on the outside bend has more energy that can cause scour, or erosion, and the meander tends to grow in the direction of the outside bend. Stream meanders reduce the slope of the streambed, and are one factor that results in a decrease of stream power. In other words, a stream with more meanders will tend to have less power than a similarly sized stream with no meanders.
Figure 1: Pattern and dimension of: (a) a meandering channel and (b) a straight channel.
Figure 2: Plan (over head) view illustration of velocity distribution in a stream reach.
Repeated erosion and deposition causes the channel to change its shape by adding more bends, and also causes the channel to migrate across the landscape over time. Figure 3 demonstrates the river's movement, or channel migration, over time. It is important to note that while the location and extent of the erosion and resulting deposition may change over time, the width and depth of a stable stream does not change much. This concept is known as dynamic equilibrium.
Figure 3: Example of the channel migration of Salt Creek, in Ohio, that occurred over 50 years. (Source: Mecklenburg et al., 2004)
Size and Amount of Sediment
Stream systems are constantly changing to balance energy (i.e., slope and flow of water) with resistance (i.e., amount and size of sediments). When equilibrium is achieved, a certain quantity of water flows at a certain speed where erosion - or cutting -- on one bank is balanced by sediment deposition -- or filling -- on the other. In other words, the amount of sediment, also called bed load supply, coming into the system is equal to the bed load supply of sediment leaving the system as water flows through it. The constant erosion and deposition of sediments that forms bends in the channel also forms places called bars where sediments build up, and other features such as riffles and pools (Figure 4). In a stable system, these features are easily identified and they are located in a predictable pattern along the channel.
Figure 4: A typical pattern (over head view) and profile (side view) for a stream channel in equilibrium.
The size of bed material (Figure 5) moving through the system is a measure of resistance of the flow of materials through the system. Larger material will have greater resistance to flow than smaller material. In other words, more stream power is necessary to move larger particles. If stream power is high enough to sweep particles up off the stream bed, erosion of the bed and banks, or degradation, will occur. If stream power is low, deposition of sediments on the bed and bank, or aggradation, will occur.
Figure 5: Various sizes of bed material found in streams.
Management Applications
Knowing what forces are acting within stream systems and how they interact with each other can help stream and watershed managers better assess stream systems and make recommendations for improvements to regain dynamic equilibrium. To assist us in understanding how discharge, slope, bed load supply, and bed material size influence each other and the stability of the stream system, we can refer to the Lane sediment balance diagram (1955). A conceptual illustration of this diagram is presented in Figure 6, where stream slope and stream discharge are proportional to sediment bed load supply and sediment size.
Figure 6: A conceptual illustration of the Lane sediment balance concept.
Background Readings
Required:
- There are no required readings for this module.
Additional Resources:
- Ward, A and S. Trimble. 2003. Environmental Hydrology 2nd Edition. Lewis Publishers: Boca Raton, FL. 475 pp.
- Lane, E.W., 1955. Design of Stable Channels. Transactions, Am. Soc. Civil Eng 120: 1234.
- The STream Restoration, Ecology, & Aquatic Management Solutions (STREAMS) Project - Multi-agency initiative whose goal is to provide education, information, technology and communication on stream management strategies. Website: http://streams.osu.edu
- "Stream Visual Assessment Protocol" http://www.nrcs.usda.gov/technical/ecs/aquatic/svapfnl.pdf