There are many different types of rivers. Experienced river restoration practitioners and lay people can immediately recognize differences between a highly confined, high gradient mountain stream and an unconfined, low gradient alluvial river. The morphology of any given river reach reflects the water and sediment inputs to that reach, geomorphic controls such as valley setting and unique watershed history. In many river restoration settings however the natural regime of water and sediment inputs have often been altered resulting in changes to channel form and associated vegetation and other biotic communities. The foundational idea behind a watershed assessment is that different types of rivers (e.g., highly confined, high gradient mountain stream vs. unconfined, low gradient, alluvial river) have different capacities to adjust to changes in boundary conditions and therefore will respond differently to restoration actions (Montgomery and Buffington, 1997). Both watershed and reach scale characteristics and processes constrain the range of possible responses to any restoration action. A common critique of many river restoration projects is that the restoration actions fail to take into account the local hydrologic and geomorphic conditions and attempts to impose forms that are incompatible with existing hydrogeomorphic processes, often resulting in failure (Palmer et al., 2005; Kondolf et al., 2001) A watershed assessment is therefore critical to understanding the constraints on restoration treatments and can help inform the actions most likely to meet restoration objectives. Furthermore it can help predict the range of potential responses which is critical in establishing realistic expectations and addressing potential uncertainties and adverse impacts. Furthermore, it ensure that resources are spent most efficiently, and as such is an important economic consideration in river restoration.
There are a number of assessment tools available to river restoration practitioners. (See Resources and Links) The objective of this section is to provide a brief overview of the characteristics/processes that a watershed assessment should cover and how their assessment is essential to river restoration in general. The topics a watershed assessment should address and the questions it should answer are listed in Table Z.
Section 1 covers the elements/topic areas that should be covered in a watershed assessment. Section 2 describes how a watershed assessment can be combined with a historical assessment in order to evaluate the recovery potential of any given river/watershed
Hydrology What is the natural flow regime? (Magnitude, frequency, duration, timing, and rate of change of flow) Geomorphology What is the valley setting (i.e., is the river in highly confined, partly confined, or unconfined?) What is the channel planform? Are floodplains present? Is there channel-floodplain connectivity? Why/why not? What is the dominant substrate? What is the natural sediment regime? Where is sediment sourced from? How much large woody debris (LWD) is present? What structural elements (boulders, wood, beaver dams) are present and what effect to they have on stream processes? What geomorphic units/habitat types are present and how are they organized? Riparian Vegetation Is riparian vegetation present? How much? How does riparian vegetation interact with different flow stages? Are there multiple age classes of riparian vegetation? Are invasive species present?
Flow has been called the “master variable” in river systems because of its influence on physical and chemical characteristics of rivers, including channel geomorphology, riparian communities, and water temperature (Poff et al., 1997; Powers et al., 1995; Resh et al., 1988). The natural flow regime (Poff et al., 1997) described five components that must be addressed to understand how flow can affect river morphology and ecosystems: 1) Magnitude 2) Frequency 3) Duration 4) Timing 5) Rate of change. Each of these factors exerts an important influence over riverine ecosystems directly and/or indirectly. Flow is responsible for shaping the physical template upon which stream ecosystems are built and it promotes or discourages certain riparian communities. Distinguishing snowmelt fed rivers from rain-fed rivers and flashy systems from more stable systems is essential to developing an appropriate restoration design. Information on the natural flow regime can be gathered from numerous sources including USGS gaging stations, regional regression curves etc. Knowing how these attributes have changed, and whether or not they can be manipulated is critical to assessing what restoration treatments are most likely to be successful and what the recovery potential is. For example, do significant water diversions upstream prevent annual high flows? Is the frequency of overbank flows altered due to changes? Are there more frequent high-runoff events due to increased urbanization of the watershed?
Geomorphology Rivers that flow through wide, low gradient valleys with high channel-floodplain connectivity will respond differently to disturbance (natural or anthropogenic) than rivers flowing through steep canyon walls with no floodplain present. A geomorphic assessment identifies the landscape controls on river behavior and uses that information to predict river response to specific restoration treatments in order to select those that are most appropriate to restoration objectives. A geomorphic assessment may rely on remote sensing data and data collected in the field. There are numerous protocols for geomorphic assessments (e.g., River Styles, more here). The type of assessment will be constrained by available resources and time, however in order to ensure appropriate restoration actions are developed a watershed assessment is essential. An abbreviated discussion of key elements of a geomorphic assessment and their importance is below.
Valley setting and channel confinement - A river’s ability to adjust laterally is a function of the valley setting and channel confinement, and therefore an important control on response to restoration.
Channel planform - Channels planform reflects key processes such as sediment supply, bed and bank materials (resistance and erodibility), importance of riparian vegetation, importance and potential influence of structural elements.
Floodplains - Floodplains are a buffer during high flow events, allowing flow to disperse laterally. They provide critical habitat to many aquatic organisms during different life stage and provide flow refugia during high flows. They are also important sites for nutrient cycling and can act as both a sink and source of nutrients and sediment. Assessing channel-floodplain connectivity requires assessment of both hydrological characteristics (magnitude of flow) and geomorphic characteristics (elevation of floodplain relative to channel bed).
Bed and bank material - The material that composes the bed and banks of a river controls the entrainment, transport and deposition of those materials. Finer grained materials are more likely than coarse grained materials to be entrained and transported. Coarse-bedded rivers are less likely to respond to alterations in water and sediment inputs than fine-bedded rivers.
Structural Elements - Features such as large boulders, LWD and beaver dams exert significant influence on physical heterogeneity in a river by locally influencing hydraulics and patterns of erosion and deposition. The ability of a reach to recruit and retain structural elements (i.e., residence time) is influenced by many factors including slope, discharge, planform, hillslope connectivity and characteristic riparian vegetation. The physical complexity promoted by structural elements increases the diversity of habitat types for aquatic and terrestrial organisms.
Geomorphic Units/Habitat types - The assemblage of geomorphic units/habitat types (i.e., pools, riffles etc.) is one way to evaluate the physical complexity of any given reach. Often restoration projects aim to increase physical complexity in order to improve aquatic habitat.
Riparian vegetation provides shade that regulates stream temperature, nutrient inputs, can be a source of large woody debris, influences the development of undercuts, increases roughness and influences patterns of erosion and deposition, and provides habitat for terrestrial insects that form the basis of aquatic food webs. (citations) Riparian vegetation is affected by and in turn affects hydraulic, hydrologic and geomorphic processes through multiple feedback mechanisms (citation). Additionally, it may be an indicator of the degree of impairment of hydrologic and geomorphic processes at the reach and/or watershed scale. An assessment of vegetation therefore is critical to understanding how it will respond to restoration treatments and how that response will in turn influence restoration treatments. Because of its influence on many stream processes (local hydraulics, patterns of erosion and deposition, recruitment and retention of large woody debris, forage and building material for beaver) riparian vegetation is critical to process-based restoration
The decision to restore a river is typically based on the idea that the current condition of the river is degraded when compared to the historical condition. An important part of a watershed assessment therefore is determining what condition the river is in relative to a historical condition and to identify the causes of degradation in order to determine the boundary conditions for current restoration efforts. Identifying the causes of degradation, whether or not, and to what extent those drivers are still operating allows restoration practitioners to set realistic restoration objectives and design appropriate restoration treatments. The ‘condition’ of a river is likely to vary throughout a watershed. A watershed assessment can inform the condition of specific river reaches by comparing reaches subject to similar hydrologic and geomorphic controls.
Evaluating the condition of a river based on a watershed assessment allows restoration practitioners to assess the recovery potential of any given reach. The recovery potential combines the geomorphic setting, hydrologic regime and the current river condition in order to determine what restored state is achievable. By recognizing historic and current drivers of degradation, and alterations to hydrologic and geomorphic processes practitioners can develop restoration treatments relevant to their specific watershed and reach context.
A watershed assessment gives river restoration practitioners critical information about the state of important physical processes operating in a watershed. River restoration projects tend to take place on a relatively short (< 5 year) timeframe, however the physical processes acting on river restoration projects operate continuously over longer time frames and are characterized by natural variability. Understanding the natural variability and the boundary conditions within which a restoration project takes place is essential for long term success.