Fluvial+geomorphology+1

=Introduction to Geomorphology and Applications to River Restoration=

//by Rebekah Levine//
=1. Introduction= Between 1990 and 2004 over $15 billion dollars were spent on river restoration projects in the United States alone ( Bernhardt et al., 2005 ), but these projects are often undertaken without strong understanding of how rivers respond to change ( Lane and Richards, 1997 ; Gaeuman et al., 2005 ). A significant problem in river restoration is determining what condition the river should be restored to, if restoration is needed at all, so that the river will function in its new “repaired” state in the years following completion of the restoration. This requires disentangling human versus natural impacts on fluvial systems ( Macklin and Lewin, 1997 ), but determining fluvial history involves understanding the processes at work that are affecting change in a fluvial system, and the temporal and spatial scales at which these processes are occurring.

Rivers studies are often conducted by taking measurements of the present channel form and processes. These measurements might include grain size distributions, cross-section surveys, bank stability or discharge and velocity. However, these measurements represent an instantaneous moment in the existence of a river, a dynamic system with a long history ( Schumm, 1977 ; Knighton, 1998 ). Using the metrics that describe the river at a specific point in time and space is a bit like looking out a train window as you pass through Chicago and imagining that you now have an understanding of the city. Given that for a specific river restoration project we do not have infinite time to observe the river and its responses to past events; what are some tools we can use that can help us better understand fluvial systems and their evolution?

Many of the tools available to understand long-term fluvial evolution can be found in the field of fluvial geomorphology. Fluvial geomorphologists study the interaction of fluvial systems with the Earth’s surface over time and space. In studying the landforms that result from earth surface processes, the field of fluvial geomorphology incorporates and informs fields as diverse as climatology, ecology, engineering and sedimentary geology. This paper will provide an overview of basic principles in the field of geomorphology that should be considered when attempting to understand the state of a river. A brief discussion of uses of geomorphology in the field of watershed, river and riparian restoration is also included.

=2. Basic Principles in Fluvial Geomorphology=

2.1. Fluvial Geomorphic Balance
The most fundamental concept in fluvial geomorphology is that a river represents the balance between the forces that drive change and those that resist change ( Lane, 1955 ). The potential energy provided to water is a driving force. Anything that is causing a loss of that energy is a resisting force. The relationship in a fluvial system between driving and resisting forces can be expressed by: Where //Q// is discharge //Sb// is the bed slope, //Qs// is the sediment discharge and //D50// is the median grain size of the bed material. At specific channel locations, the driving and resisting forces can be understood by investigating hydraulic relationships. Relationships such as the Reynolds number parameter, the Manning equation and the Chezy equation all take into account the effects of resisting forces on flow. Differences in conditions at the boundaries of flow – the rocks, sediment and bank material – affect the response of the water flowing downhill under the force of gravity. Variation in shear stress between laminar and turbulent flows is useful in explaining the effects of resistant and rough channel boundaries. Shear stress for laminar flow is:



Where µ is viscosity and //dv/dy// is the velocity gradient or rate of change of velocity (v) with depth (y). Shear stress for turbulent flow has an additional term, //n//, which is the eddy viscosity representing the fact that not all flow proceeds in straight paths, so that the apparent viscosity increases : Most stream energy, where flow is dominated by turbulence, is dissipated by eddy viscosity. Excess energy that is not dissipated by flow turbulence is available for sediment transport and erosion. When the sediment load provides more resistance than there is stream power to transport it, streambed aggradation may be the result. However, high suspended sediment loads can dampen turbulence, and thus decrease eddy viscosity, increasing flow velocity and erosive potential ( Ritter et al., 1995 ). Another way to understand the balance between driving and resisting forces is to use Bagnold’s ( 1977 ) stream power approach. The total power per unit of stream bed is: where, Q and S are specific weight, discharge and slope respectively and is shear stress while are shear stress and is average velocity. Where the available stream power is greater than that needed to transport the sediment load, erosion of bed and bank material is the result. When there is just enough stream power to move the load of sediment then the stream is in equilibrium and there is no net deposition or erosion occurring.

Within a river system variables that are resisting the stream power include sediment size, hydraulic roughness, bank cohesiveness and variables that drive change include discharge (//Q//), which is the volume of water per unit time, slope (//S//), sediment supply (//Qs//) and sediment size. Understanding the interaction between these variables is complex because not only is the fluid in the channel constantly deforming and adjusting, but the channel shape itself is adjusting to changes in flow. Neither fluid nor container remains constant. Table 1 summarizes channel response to changes in discharge (//Q//) and sediment load (//Qs//). The work conducted by Schumm (1977) continues to be refined as additional research in both natural channels and flumes sheds light on how channels respond to adjustments in discharge and sediment supply (Gaueman et al., 2005; Madej et al., 2009 ).

Table 1. Table adapted from Schumm (1977) showing adjustments to channel attributes with changes in discharge (Q) and sediment supply(Qs). (+) indicates increase, (-) decrease, (±) could increase or decrease (c) variable remains constant, (na) no response or unknown response.

The majority of these variables are responding to changes in stream power. A slight increase or decrease in stream power has the potential to change sinuosity ( = channel length / straight-line valley length) in a stream reach ( Schumm, 1977 ) or impact the rate of bank erosion ( Lawler et al., 1999 ). The type of channel form that will develop in response to a given stream power depends on the forces that are resisting change, in particular, bank material and basal stream material ( Simon and Rinaldi, 2006 ). This concept is important in thinking about restoration, because incision is often one of the reasons that restoration projects are initiated ( Zaimes et al., 2006 ; Florsheim et al., 2008 ). A primarily cobble bedded stream will respond to moderate increases in discharges by eroding their banks rather than incising ( Simon and Rinaldi, 2006 ). Large discharge increases, however, may cause cobbles to move and even a cobble bedded stream will respond by incising ( Madej et al., 2009 ). Bank resistance is affected by the percentage of silt versus clay in the banks and the presence of vegetation. Resistant bank material will decrease rates of river migration ( Hudson and Kesel, 2000 ; Brooks et al., 2003 ), while lack of resistant material will inhibit meander development ( Hooke, 2003 ). Whether a channel responds to an increase in discharge by incising or widening depends on the ratio of resistance between bed and bank material ( Simon and Rinaldi, 2006 ).

2.2. The balance in time and space
In the above discussion, we have been exploring the balance of driving and resisting forces, but when trying to understand an entire river system two other variables are important to keep in mind: time and space.

2.2.1.Time
Time is a means of measuring change. It can be viewed as a proxy for the rate of energy expenditure, work done or change in entropy ( Schumm, 1991 ). Change on a short time scale may be insignificant if the focus is on long term evolution of a system. For example, an individual storm event can increase discharge and the ability of a stream to transport sediment, but this change may not result in a significant adjustment of channel form. However, the location of individual pools and riffles may change. Small scale changes set in motion by this storm event are not pushing the system over any thresholds and into a new system state and so, on intermediate time scales (100 – 1000 years), the storm has not changed the overall balance of driving and resisting forces in the system. Following the storm event the general form of the river will be maintained. However, if increased precipitation becomes the norm over a decade, average discharges will increase, likely leading to adjustments throughout the system ( Table 1 ). The length of time a disturbance persists and the recovery time of the system determine how large the effect of a given disturbance is. If, for example, the frequency of large rainfall events increases, and the channel no longer has time to narrow its channel boundaries prior to the next large event, then the channel may have a new wider equilibrium form, adjusted to carry frequent, large rainfall discharges. The shift from one channel form to another may happen instantaneously as the stability limit of the established channel form is crossed ( Schumm and Lichty, 1965 ; Ritter et al., 1995 ).

The variables that can change within a watershed include characteristics of the flow, channel morphology, mean water and sediment discharge, vegetation, soil/regolith, slope morphology, regional relief and the geology. Depending on the time scale of interest, variables can be considered dependent, independent or irrelevant (Table 6.1 from Knighton 1998 p. 263). In river restoration, the time scale of interest may span instantaneous to medium time scales with humans contributing to change on short and medium timescales primarily through land use practices. Although human impacts may not extend to longer time scales, the river might display morphologic characteristics inherited from past events. For example, on the South Fork Payette River, Idaho the channel is still adjusting to the decrease in sediment supply from the last glacial period by actively incising ( Pierce et al., 2011 ). An example on human timescales can be found from mining impacts in California that began in the 1850’s and are still promoting aggradation in affected streams ( James, 1999 ). Understanding that response times in fluvial systems can be slow may aid in understanding perturbations that are influencing the present fluvial form and inform management decisions.

2.2.2.Space
Table 1 shows that channel response can occur at a specific site (width and depth adjustments) or be felt along the length of the channel (slope and meander adjustments). Each site along a river corridor is related to all other sites within the watershed, so an adjustment in channel width at one location is likely related to processes occurring in the watershed at other locations. The location of a given reach within the channel network whether it be headwaters, midsection or lower basin, may help dictate how a stream should function in a given reach ( Schumm, 1977 ). Energy gradient changes downstream and is directly responding to hillslopes, channel gradient, discharge and sediment supply. This gives reaches distinct disturbance regimes ( Montgomery, 1999 ; Wohl et al., 2005 ).

When thinking about spatial variability, it is important to keep in mind the changes occurring at a variety of spatial scales. Just as change can occur at various time scales it can also occur at various spatial scales from bed morphological features such as riffles and pools to channel reaches and the entire watershed. And just as with time, events at one spatial scale may not be important at another. Often events occurring at large spatial scales, uplift of the drainage basin, a climatic shift or lowering of base level downstream, are also occurring at large temporal scales.

Time and space cannot be divorced from one another in understanding fluvial systems and response to perturbations. Work by Schumm and Parker ( 1973 ) showed that rivers respond to change in a complex way and that change is not felt at all sites simultaneously. Using a laboratory model of a river system they adjusted baselevel and watched how the system responded (fig). The system responded immediately near the river mouth by incising, but it took time for the effect of the baselevel to propogate upstream. The upstream movement of the knickpoint added sediment to the system, so that as upstream sites were incising, downstream sites were filling with the upstream sediment. The response of this system was non-uniform in time and space. Schumm and Parker ( 1973 ) termed this phenomenon complex response. Figure 1. From Schumm and Parker (1973) - Diagrammatical cross-sections of experimental channel 1.5 meters from channel outlet (base level). //(a)// Valley and alluvium deposited during prior run (b) After 10 cm lowering of base level, channel incises forming a terrace, (c) Inset alluvial fill deposited as sediment from upstream increases (d) a second terrace is formed as channel incises again creating low width-depth ratio channel with reduced upstream sediment load.

=3. Records of space and time= How can the principles of time, space and the geomorphic balance be used more practically to understand a river system? The landforms and channel planforms that exist within the river corridor reflect the balance of the driving and resisting forces over various spatial and temporal scales. Investigations of the present channel state may include measuring the channel longitudinal profile, analysis of channel planform and multiple channel cross-section surveys along the length of the stream. Each of these analyses takes into account spatial variability as well as complex response, so that it may be possible to determine the stage of channel evolution of the system ( Simon and Rinaldi, 2006 ). In order for the measurements of profile, planform and cross-sections to be the most useful they also need to be understood temporally. Have there been repeat surveys of the river? Having a plan to do so will make it possible to explore trends in knickpoint migration or channel filling. Cross-section data from stream gauges may also be useful for collecting a dataset that expands across several decades. Old maps and air photos are resources that allow investigation of planform changes ( Knighton, 1998 ). Aside from the measured variables above, mapping and identification of landforms within the river corridor also provide information on the long-term evolution of a river system.

Investigations of terraces and terrace sequences can be useful in understanding fluvial response on large timescales (e.g. Bull, 1991 ; Meyer et al., 1995 ). Terraces, at the very least, are landforms that represent periods of aggradation and incision by any number of processes with major terraces usually the result of climate or tectonic events and minor terraces resulting from complex-response ( Bull, 1990 ). They can also represent periods of vertical stability which is the case with bedrock strath terraces and fill-strath terraces where formation is primarily by lateral planation ( Bull, 1990 ). These straths are often covered by thin deposits, where deposits that are less than or equal to ~ 2 times the average bank height do not constitute aggradation, but represent a vertically stable stream ( Pierce et al., 2011 ). The timing and potential cause of the events that promoted terrace development can be investigated using soil development, pairing of terraces and absolute dating of organic material in terraces ( Bull, 1990 ). Other distinctive characteristics of the sediment, such as grain size and sorting are also considered when making correlations across, or down, the channel. There may also be a distinctive layer, such as a tephra unit, to aid in distinguishing a given terrace level ( Meyer et al., 1995 ). Soil development and terrace elevation can provide relative ages while radiocarbon, optically stimulated luminescence and dendrochronology can provide absolute dates.

Paleochannels are another useful feature that can aid in interpreting fluvial processes (figure 2). Since rivers devoid of human impact are extremely rare ( Wohl, 2006 ), finding references to compare pre and post-disturbance is difficult. The use of the long term fluvial record (paleochannels) is one way to get around this investigative problem (e.g. Starkel, 1995 ). Paleochannels can be used to derive paleosinuousity, paleochannel planform and possibly allow calculation of paleoflows (e.g. Carson et al., 2007 ). The radiocarbon dating of material in channel fills can help constrain the age of channel abandonment.

Figure 2. Paleochannels filled with snow. Photo is taken from the top of Nemesis Mountain, just east of Hell Roaring Canyon, Centennial Valley, MT. The modern channel is on the bottom right in this photo and is flowing north, away from the viewer (photo courtesy of Hayes Buxton, USGS).

=4. Applications to River Restoration= The process of looking upstream and downstream as well as back through time allows practitioners to understand the natural range of variability within watersheds as we think about their long-term function ( Wohl et al., 2005 ). An understanding of the natural range of variability in stream systems allows for the development of flexible restoration and management plans that incorporate what is known about past river function. A river restoration design that incorporates the observed variability and ongoing processes in the river is likely to be more successful ( Kondolf, 2006 ). Assessments of an individual river system that incorporate how driving and resisting forces interact across time and space allow for the identification of the cause of degradation rather than just addressing the observed symptoms at a specific site. Restoration projects that include geomorphic assessment are more likely to find the source of the problem (e.g. road building in the headwaters) and target that rather than spending money downstream at the source of the problem ( Wohl et al., 2005 ).

Understanding that rivers reflect the balance of driving forces (stream power) and resisting forces (e.g. geology, vegetation) shows that each drainage system is likely to have a unique combination of these variables, so that assessments of river form and function need to be done at each site prior to undertaking a project. The general principles of geomorphology can be applied, but blanket assumptions about river process may lead to unsuccessful projects ( Wohl et al., 2005 ). For example, a river flowing through shale will quickly downcut and erode away any relief in the landscape whereas gneissic bedrock landscape may create a very different channel network, even if the climatic variables are constant between the two. If a restoration project has the goal of making the shale stream look like the gneiss stream then it will be unsuccessful because the resisting forces in the two cases are different. Another example of projects misunderstanding landscape variables involve projects with aesthetic goals rather than process goals. This has particularly been an issue with many people’s obsession with meandering single thread channels ( Kondolf, 2006 ).

Restoration projects that understand the natural range of variability and the different processes occurring in different regions are likely to be more successful than those that are interested in channel form at a few specific locations, or those that do not understand natural variability in flow, vegetation dynamics and landscape disturbance. A successful project in the planning stage may include posing questions such as: What is the source of the problem? Can we change something at the problem site rather than re-engineering the stream? What is the natural variability that historically created desired habitat? What has changed that has limited the creation of that habitat? Are the project goals to reestablish habitat forming processes or is the project designed to function successfully with the present altered habitat forming processes ( Montgomery and Bolton, 2003 ). Another important question to ask is can the perceived problem be solved by human intervention? An example where intervention may not be successful is prevention of stream incision at sites that are still responding to regional uplift or shifts from an ice age climate. If these questions are posed, and there is flexibility built into project goals, then it is likely that success rates will be higher than projects that do not consider geomorphic principles of time and space.

=5. Conclusions= Geomorphology with its unique view of temporal and spatial scales provides an essential toolkit for successful restoration projects. Measurement of biological integrity can be difficult and hard to assess on reasonable time scales ( Wohl et al., 2005 ), but an understanding of processes and disturbance regimes may lead to successful maintenance of essential habitat ( Montgomery and Bolton, 2003 ). A strong understanding of geomorphic principles and fluvial processes will allow projects to be more successful, but strong interdisciplinary teams that include engineers, stakeholders, geomorphologists and biologists are likely to meet with the most success. Rivers represent the intersection of human history, landscape history as well as aquatic and terrestrial ecosystems. Making sense of these cross-roads requires interdisciplinary effort.

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