“Stewardship of rivers… today faces a distinct but universal challenge: meeting the needs of increasing human use while accounting for and preserving the natural variability and ecosystem function to the greatest extent possible. Historically, the full range of values associated with healthy rivers was not understood or appreciated… There was a tendency to view the many components of the riverine systems as individual rather than interconnected parts of a complex cycle. This perspective led to unrestrained development of groundwater and surface waters even though these are two of the smallest components of water on Earth.” (IFC, pg 1)
The Five-Component System
The five-component system of evaluating watershed health is the cornerstone of the Watershed Health Assessment Framework. Hydrology, Connectivity, Biology, Geomorphology and Water Quality – one component alone cannot describe a watershed system. By studying each component in context with the others, an understanding of the system begins to emerge.
Hydrology is the component that drives the system. The continuous hydrologic cycle transfers water, energy, nutrients and matter throughout the watershed. The power of moving water, whether a raindrop displacing soil, or a flooding torrent reshaping the floodplain; keeps the system in a continuous state of flux.
Within the stream, hydrology refers to the source, amount, and rate of water, both spatially and temporally, in a stream channel. It impacts the development of aquatic and riparian vegetation, microhabitat features, as well as the other four components. Human activities such as draining wetlands, building roads, creating tile lines, and withdrawing water will alter stream hydrology and affect the hydrologic cycle within the watershed.
Connectivity is the component that holds the system together. It refers to the flow, exchange, and pathways of organisms, energy, and matter. Connections exist on all scales and in all directions. Connectivity often refers to the river’s ability to access floodplains and wetlands during high water, but watershed connectivity also refers to terrestrial habitats being functionally and spatially accessible at the time they are needed for life cycle completion.
There are physical barriers to connectivity such as dams, but flow reduction from water withdrawal, chemical barriers such as zones of poor water quality, or biological barriers such as competition from invasive species or fragmented microhabitat can also have disruptive effects.
Biology is the component that describes the web of life within the watershed. Aquatic plant and animal species require various habitat components at various points in their lifecycle. The importance of pools, riffles, and runs are well studied for fish communities, particularly game fish. The presence, amount, and arrangement of these microhabitat features are directly related to the river’s hydrology and geomorphology. Water quality and connectivity also impact the presence and persistence of aquatic species. Additionally, the mosaic of terrestrial plants and animals along the stream, in its floodplain, and in its valley are vital to the character of the watershed. Plants are critical components of nitrogen, carbon, and oxygen cycles, serving as production sites and conversion centers for life-sustaining elements. Throughout a stream’s length, the vegetation along the riparian corridor intercepts flows of incoming runoff, nutrients, and contaminants.
Rather than attempt to quantify the complexity and scope of all the biological components within a watershed system, key indicator species and habitats are used to give an indication of abundance and diversity within the watershed.
Geomorphology speaks of the geologic template of landscape topography, soil type and stream flow patterns that lay the foundation for the other components. The shape of the stream channel itself, such as meanders, oxbows, backwaters, and floodplain reflects the dynamic nature of the stream system. These forces interact to create and maintain diverse habitat for many aquatic species. Stream geomorphology directly impacts a river’s hydrology and water quality as well: An unaltered, complex geomorphology helps to attenuate sedimentation and river flooding downstream.
These landscape level processes interact with human land use patterns to drive the system in new and often unexpected directions. Quantifying and describing these relationships moves us toward an understanding and an ability to anticipate the consequences of land use decisions.
Water Quality is the component that describes the current condition of the water in the stream. It often refers to the stream’s chemical balance, water temperature, sediment load, chemical pollutants, and nutrient load. Aquatic species may be adapted to a certain set of water-quality conditions such as needing coldwater streams, or may be deleteriously impacted by unnatural water quality components, such as the input of estrogen-mimicking compounds.
Sediment and contaminant impaired waters are a symptom of unhealthy contributions from the surrounding watershed. These can be direct discharges into the stream, or mobilized from elsewhere in the watershed as a result of land use practices and carried to the stream. (Adapted from MN DNR 2006, Tomorrow's Habitat, Rivers Overview pg 275.)