River restoration is an interdisciplinary practice that integrates physical, biological and social sciences. In some cases those interested in implementing river restoration projects may not have an extensive background in these areas. The purpose of this section is to introduce some fundamental concepts and terms that are required in order to understand the approaches described throughout this site. This section focuses on the physical sciences, the “nuts and bolts,” required to understand how rivers behave and respond to different actions. In addition to a short explanation of each of the following concepts, its relation to cheap and cheerful restoration work is briefly described.
There is a rich and extensive literature on the processes that shape and are shaped by river and floodplain ecosystems. For more information readers are directed to the References page.
River are dynamic systems whose form can change over time. Over short time scales (1-10 years) river form may appear unchanging, or static, such that an observer visiting a particular reach may easily recognize it to be the same, its form nor location having changed. Over long (100-1000 yrs) time scales, rivers may adjust to change in response to changes in climate, land use or other disturbances (e.g., fire, urbanization, extirpation of beaver). Equilibrium means that the net amount of sediment being delivered to a certain reach equals that transported out of the reach. It does not mean that the river will not move across the landscape. Unconfined meandering rivers illustrate how a river that has not been influenced by any natural or human caused changes will migrate laterally across its floodplain. While the precise location of the river may change, its behavior and the processes at work are fundamentally the same. Lateral migration is a combination of erosion that takes place on the outside of a meander bend and deposition that takes place on the point bar on the inside of the meander bend. Such processes are critical to creating and maintaining instream and floodplain habitats. Evidence of past migrations is easily viewed in satellite imagery available from such platforms as Google Earth or Google Maps.
Photo of the Rio Negro, Argentina taken from the International Space Station. The floodplain, roughly 10 km wide, preserves the record of historic location of the river in former meander bends.
Systems such as those described above are not stable in the sense that their form is unchanging, rather their characteristic form and the processes that influence them vary around a mean. Designing appropriate river restoration projects requires that we understand this natural range of variability of intact river systems. Furthermore, we advocate for restoring the conditions that will allow a river to adjust naturally to its water and sediment regime, rather than imposing a predetermined form upon it.
Ecologists recognize the role of disturbance as critical to maintaining a mosaic of habitats that promotes diverse and resilient river and floodplain ecosystems. The regular disturbances experienced by many rivers are critical to creating and maintaining the diverse habitats that support ecologically healthy communities.
The flow regime describes the characteristics of flow that influence the physical processes of erosion and deposition as well as the riparian and floodplain communities they support. The flow regime of any given river system is characterized by five attributes (Poff et al., 1997):
Designing appropriate restoration projects depends on understanding how the natural flow regime and/or altered flow regime will interact with restoration. In many cases, such as below dams, an altered flow regime is responsible for the degradation of quality habitat, by limiting peak flows, decreasing sediment delivery and altering the timing and duration of flows of a given magnitude, though special attention is often paid to peak discharges.
Geomorphologists, hydrologists and stream ecologists all seek to describe riverine attributes that are important to restoration goals. Often, increasing pool-riffle sequences, or backwater habitats are important restoration goals. Geomorphic units are topographically defined features that make up riverine and floodplain environments and are defined by their shape and location within the river corridor. Pools, riffles, steps, backwaters, point bars and floodplains are examples of geomorphic units. Surveying the assemblage of geomorphic units present along restoration reaches helps practitioners describe the current conditions, potential reference conditions, and desired restoration outcomes. See Wheaton et al. (2015) for a complete description of geomorphic units.
Structural elements are discrete elements that exert a direct influence on stream hydraulics and therefore patterns of erosion and deposition. Structural elements often cause flow separation, backwater effects and convergent flow that creates physical complexity. Examples of structural elements include rock outcrops, large woody debris and beaver dams as well as man-made elements such as rip rap, bridge piers etc.
The valley margin is the margin between a bedrock hillslope and the alluvial sediments that make up the valley floor. The valley floor can be composed of contemporary floodplains, historic floodplains (terraces) and alluvial fans.
The valley bottom margin is the margin the divides the active channel and contemporary floodplain from other valley floor features such as terraces and alluvial fans. In general the valley bottom margin is the margin that defines whether a stream is confined, partially confined or unconfined which exerts a major control on river behavior.
Stream power is a measure of the amount of energy flow exerts on the channel bed and banks. It can be used to help predict how a river will behave in response to restoration practices, or whether or not certain structural elements, such as LWD or beaver dams will persist during high flows.
It is defined as:
Stream power = Density (H2O)* g* Slope * discharge
such that rivers with higher slopes will be characterized by higher stream power, and for any given river, stream power will be higher during higher flows.
It is important to note that stream power changes from the headwaters to the mouth. In other words, stream power is measured at a single point along any given river.
Stream power is defined as:
Ω = γQS
Rivers tend to have characteristic forms based on the climatic, geologic and geomorphic setting in which they occur. The natural range of variability for a given river describes the range of morphologies that may occur rather than a single, static form. These morphologies are a product of the processes that act on the river system that have variable magnitudes and frequencies. As a result a given reach of river may exhibit multiple forms through time. The challenge for river restoration is identifying what conditions exist within the natural range of variability and what states represent a departure from the natural range, and are therefore in need of restoration. This information is best assessed through watershed assessments that identify current and historic climatic, geologic, geomorphologic and anthropogenic control on river behavior.
A river can be classified as degraded if it does not possess by the physical and ecological processes that characterize its unaltered state. However such a state may also be ‘stable,’ such that the river is unlikely to change its condition in the absence of external disturbance or restoration. In some cases the degraded state may be in equilibrium (neither eroding or aggrading). In such cases restoration practitioners need to identify the historic condition and determine an appropriate goal for restoration. The goal may be a return to the historic condition or a new condition based on current constraints. For an example of alternative stale states and river restoration see: Wolf et al Marshall et al-double check these
Channel incision is a widespread problem that influences a number of physical processes that lead to the degradation of aquatic, riparian and floodplain ecosystems. Channel incision lowers the elevation of the streambed, resulting in lowered channel-floodplain connectivity and lowered water tables, which limits water availability for riparian plants. Because of its effect on water tables, incised channels can reduce baseflow discharges which can result in warmer temperatures and decreased habitat quality. These consequences, in turn, prevent the continued recruitment and growth of a healthy riparian corridor which further reduces shading and instream wood recruitment furthering ecosystem degradation. As streams incise and channel floodplain connectivity decreases, the energy of high flow events is concentrated on the channel bed and banks, which can lead to further incision or bank erosion.
Mulitple channel-incision evolution models have been proposed. See Schumm, Pollock, Cluer and Thorne.
While stream incision is a natural process, in many instances it is the result of land use changes, or direct alteration of the flow regime or river channel.
It is important to recognize that in both channel evolution models the stream eventually returns to a state of dynamic equilibrium. The question for river restoration is whether or not the costs associated with degradation and the time scale on which natural recovery can take place are great enough to justify a more active intervention.
Within a basin there are a number of processes and attributes that exhibit a general trend from headwaters to mouth. Among these are trends in gradient, channel width, channel substrate, channel-floodplain interactions, stream power, riparian communities and temperature. Understanding the location of potential restoration reach within the larger context of a watershed helps 1) prioritize restoration treatments 2) design appropriate restoration and 3) develop realistic objectives and expectations.