The term watershed health refers to the holistic condition of freshwater ecosystems within a watershed (EPA, 2013). A healthy watershed is characterized by having natural hydrology, biology, chemistry, and exhibiting headwaters, wetlands, floodplains, and riparian corridors, supporting diverse aquatic ecosystems and numerous biotic communities. Sediment transport, fluvial geomorphologic and natural disturbance regimes are also properties associated with healthy watersheds (EPA, 2013). However, factors such as large, intense wildfires, storm-water runoff, as well as point and non-point sources of pollution, along with the quantity of pollutants released into the environment significantly impact water quality (NOAA Watershed flooding and pollution; EPA, 2015).
Some comprehensive forest watershed monitoring efforts have taken place in California. These include: the Caspar Creek watershed study (spanning more than 50 years), Redwood Creek studies, Teakettle and Kings River Experimental Watershed (KREW) study, SNAP, Little Creek study on Swanton Pacific Ranch.
The California Integrated Watershed Assessment (2013) identifies six attributes that can be used to describe watershed health (EPA, 2013):
- landscape condition
- geomorphology
- habitat
- water quality
- hydrology
- biological condition
These attributes can be further grouped into watershed condition indicators (elements that depict the physical setting of the structural watershed as it relates to aquatic ecosystem conditions), and stream health indicators (represented by factors describing in-stream conditions such as hydrologic and geomorphic factors, habitat and biological attributes, and water quality) (EPA, 2013). A healthy biotic community within a stream is characterized by native aquatic species integrated across trophic and functional levels, exhibiting high diversity and abundance of various functional groups. In a healthy and resilient ecosystem, these species are able to adapt to short and long-term variations in environmental conditions (EPA, 2013).
In the process of conducting an ecological assessment, whether the analysis is focused at the stream level, or at the watershed level, water quality remains the basic property on which many other ecosystem health indicators are dependent upon. Under natural conditions, stream water chemistry varies within a characteristic natural range of values. Indicators that are meaningful for diagnosing water quality condition in forested settings, and are sensitive to forest disturbance should be included in ecological performance frameworks. Some important water quality indicators are described below:
Turbidity – This indicator is represented by the distance that light can penetrate a body of water. The presence of sunlight is a vital element, providing the energy required for photosynthesis, and defining the depth of the photic zone (area in the water column in which algae and other plants thrive under the exiting light regime). A change in water clarity could significantly impact the aquatic ecosystem, including impacts to fish feeding and spawning. Heavy rains may trigger increased turbidity, and lower visibility by stirring sediment and debris within the water column (Lower Colorado River Authority – Water Quality Indicators).
Water temperature – Temperature influences many water parameters such as dissolved oxygen, the plant and animal species found at a particular site, and the susceptibility of organisms to parasites, pollution, and disease. Since aquatic organisms have optimal temperature ranges, a shift in normal temperatures can affect their health and distribution. Changes in weather conditions, groundwater inflows, or shade and discharges from anthropogenic sources can cause significant shifts in water temperatures (Lower Colorado River Authority- Water Quality Indicators).
Suspended sediment - Suspended sediment is composed of silt and clay sized particles less than 0.1 mm in diameter, that are suspended in the water column. The unit of measurement is sediment per unit volume of water (mg/L). The suspended particles are transported by flowing water, and get settled when flow is insufficient to keep them in suspension. Their concentrations usually peaks during periods of increasing flow associated with intense or long duration rainfall events (RAMP – Regional Aquatics Monitoring Program).
Stream channel condition - This characteristic determines a river’s channel form in terms of pattern, profile and dimension. Knowledge of the history and ongoing trends of channel characteristics, such as water, sediment, woody debris, heat, and nutrients, is essential to effectively monitor and interpret channel condition (Taylor, 1999). Stream channel condition is influenced by watershed inputs such as water, sediment, and organic matter, as well as valley geomorphologic characteristics (such as soils and vegetation, terrain slope, channel width and underlying geological formation) (Milone & MacBroom, Inc. 2008). Even though disturbances such as forest fires and floods naturally occur, maintaining the balance in the natural system is extremely important in supporting a functional wildlife habitat, and maintaining the structural integrity of human built infrastructure (such as roads). Based on the level and intensity of disturbance events, new ecological niches may be created, enhancing aquatic biodiversity. In the case of extreme levels of disturbance, the stream channel may undergo a transition and adjustment to a completely new form (EPA, 2015). Dams and significant water diversion structures significantly affect stream channel conditions.
Age, structure and composition of riparian forest – Riparian vegetation plays a critical role in providing for a healthy stream system. Analyzing the age, structure and composition of riparian forests is important since this vegetation fulfills numerous functions: it slows the rate of runoff and minimizes erosion by protecting stream banks and floodplains from the erosive force of water. It also captures excess nutrients carried from the land, regulates water temperature changes, and conserves soil moisture, ground water, and atmospheric humidity, while also providing a source of food and cover to terrestrial and aquatic fauna (Hudson River Sloop Clearwater).
Large woody debris (LWD) loading – The presence of large woody debris performs numerous essential functions within stream systems, particularly low gradient streams without boulder/bedrock channels. It is instrumental in storing sediment and fine organic matter, it impacts channel morphology, influences the composition of riparian vegetation, and determines nutrient dynamics (Berg et al., 1998). LWD also plays an active role in shaping channel morphology, inorganic sediment storage, and influencing fish habitat, since it is often responsible for the formation of pools in forest channels (Berg et al., 1998; Taylor, 1999). Channel sediment accumulation may increase from either increased sediment loading or, as a result of increased storage potential created by woody debris (Taylor, 1999). Logging operations can act as a catalyst either increasing or decreasing the supply of large woody debris in forest streams (Taylor, 1999). Therefore, evaluation of baseline conditions of woody debris frequency and volume based on stream type is an important first step in the design of effective ecological monitoring schemes.
Road density - It is widely recognized that roads are major contributors to stream sediment loading (USDA Forest Service, 2010). Road density is often used as an indicator of watershed condition since it influences the streamflow and the hydrological integrity of the stream network (Booth and Singer, 2009). Forest infrastructure such as roads, trails, and dams often impact wildlife through habitat fragmentation, while also restricting the passage of aquatic organisms (USDA Forest Service - Watershed Health and Riparian Areas). In addition to this, roads impact watersheds by compacting soils and increasing runoff (USDA Forest Service - Watershed Health and Riparian Areas). Due to the numerous environmental impacts resulting from forest infrastructure developments, road decommissioning is viewed as a way to improve watershed conditions. Thus, the reduction in road density within a watershed is desired as part of a Forest Plan (USDA Forest Service - Watershed Health and Riparian Areas). The road density within a specified distance of a watercourse is a metric often used in monitoring watershed conditions (e.g., 200 feet or 100 m, and is referred to as riparian road density).
Road crossing density - Road crossing density (the number of road-stream crossings in the catchment divided by the catchment area) is included as an indicator of watershed condition because it influences the upstream/downstream connectivity of stream habitats (EPA, 2013). While natural systems allow aquatic species to migrate freely throughout a stream network, the construction of road-stream crossings not only disrupts the longitudinal connectivity (upstream/downstream connectivity), but also highly fragments the natural habitat (EPA, 2013). In addition to this, the construction of culverts can change stream velocity limiting fish passage, and thus altering the natural channel geomorphology (EPA, 2013).
Index of biotic integrity (IBI) and bioassessment based on benthic macroinvertebrates (BMI) - Aquatic organisms are often used as indicators of ecological condition in streams and rivers (Rehn et al., 2008). Increasingly used in water quality monitoring programs, bioassessments use datasets composed of benthic macroinvertebrates (BMI). These datasets contain information on the characteristics of species found in a sample, along with their relative abundances. Indices of biotic integrity (IBIs) are derived from these datasets and are used to measure the biological condition of an ecosystem and its response to changes in water chemistry, human-caused landscape disturbances, and changes in stream channels, and riparian zones (Rehn et al., 2008). Particle size frequency distributions can provide valuable information about instream habitat conditions that affect BMI distributions (SWAMP Bioassessment Procedures, 2007). In addition to this, coarse particulate organic matter (CPOM) is a general indicator of the amount of organic matter available at a site. This includes particles of decaying organic material such as leaves that are greater than 1.0 mm in diameter. Measuring CPOM can provide valuable information about the basis of the food web in a stream reach (SWAMP Bioassessment Procedures, 2007).
Escherichia coli (E. coli) - E. coli is a fecal coliform bacteria that comes from human and animal waste, and is used to determine whether freshwater is safe for recreation. Elevated levels of E. coli are often associated with disease-causing bacteria, viruses, and protozoans. Flooding events can cause elevated levels of E. coli.
Nitrate concentration - Nitrogen is a vital nutrient required for the growth of all organisms. In fact, in forest with salmon streams, returning spawners transfer significant amounts of nitrogen (and phosphorous) from the ocean environment to the freshwater and terrestrial forest ecosystem, making both of them more productive than non-salmon forests. However, excessive amounts of nitrates increase algae growth. This in turn can consume all the dissolved oxygen from the water column, and lead to massive fish kills, while also impacting other aquatic life. In addition to this, high levels of nitrates found in drinking water can be extremely toxic to humans. Some of the most common sources of nitrates result from human and animal wastes, and industrial pollutants. Other predominant nonpoint-sources include runoff from heavily fertilized croplands and lawns (Lower Colorado River Authority- Water Quality Indicators).
Dissolved oxygen (DO) - The amount of oxygen dissolved in the water column measured in milligrams per liter (mg/L), is vital for both plants and animals. Low levels of DO can be harmful to fish, and other aquatic organisms.
pH – Factors influencing water pH levels include: acid rain, pollution from accidental spills, agricultural fertilizer runoff, and sewer overflows. Water has a buffering capacity to resist changes in pH, however, increased level of pollutants may pose a serious threat to the survival of aquatic life since extreme values on either end of the scale can be lethal to most organisms (Lower Colorado River Authority- Water Quality Indicators).
Specific conductance: Conductivity, known as the ability of water to pass an electrical current is determined by inorganic dissolved solids such as: chloride, sulfate, sodium, and calcium, elements are derived from, and are directly affected by the bedrock geology of the area through which the water flows. Streams that run through granite bedrock will have lower conductivity, while streams that flow through limestone and clay will have higher conductivity. Organic compounds affect the conductivity of water, and warm water has a higher conductivity. Low water flow conditions, and extended dry periods also contribute to higher conductance (Lower Colorado River Authority – Water Quality Indicators).