Impervious Surface 

Background: Impervious surface refers to hard surfaces that cannot absorb water. High levels of impervious surface increase runoff into streams, reducing water quality and impacting stream communities (Thomas et al. 2013). 

Data: GIS data were used to calculate the percent of land cover in each watershed that is covered by impervious surface. After exploring local impervious surface datasets and the most recent USGS National Land Cover Database (NLCD), released in 2021 using 2019 satellite data/imagery but at a 30x30m resolution, we chose instead to use the Chesapeake Conservancy high resolution Land Use Data Project, allowing 1mx1m resolution using 2013-2014 aerial imagery. 

Threshold and scoring: Various thresholds have been suggested for impervious surface, with impervious surface covering less than 5 to 10% of land considered “protective of stream conditions” (Cuffney et al 2010). Some studies (Uphoff et al. 2009, Arnold and Gibbons 1996) indicate that anywhere from 20-30% cover by impervious surface negatively impacts natural resources, while others consider 30% to be the threshold for negative impacts.  

For this study, a threshold of 5% cover by impervious surface was selected for a score of 100% and 30% cover by impervious surface was selected for a score of 50%. Cover between 5 and 30% was scored along a scale from 100-50%. Impervious surface cover exceeding 30% was scored along a scale where 30% cover resulted in a score of 50% and 100% cover in a score of 0%.  

Fish Index of Biotic Integrity (FIBI)

Background: Fish community health (integrity) is a commonly used indicator for stream health. The Fish Index of Biotic Integrity (FIBI) is commonly used to assess fish assemblage health, and has been adapted for various environments and regions. The index considers the numbers and types of fish that make up a stream community. (Thomas et al. 2013).

Data: Data were obtained from various sources. Many organizations and groups monitor FIBI at various locations. Some locations are monitored frequently over time, and others less frequently. For each watershed, FIBI data were obtained from multiple sampling locations. For each sampling location with data, only data from the most recent year sampled were used. 

Rock Creek data were available from the Maryland Biological Stream Survey, which scores fish community health on a scale of 1-5, where 1 is the poorest score and 5 is the best. For Sligo Creek, data were obtained from the Maryland Biological Stream Survey and Montgomery County, which also scores fish community health on a scale of 1-5, where 1 is the poorest score and 5 the best. Accotink data were available from a “Stressor Analysis Report” prepared by the Interstate Commission on the Potomac River Basin. In this report, FIBI was scored on a scale of 13-29, where values equal to or less than 13 are very poor and values greater than or equal to 29 are considered excellent. 

Threshold and scoring: The FIBI is a scaled score of fish community health, so no set threshold was used. For each site with data, the provided FIBI score was converted from the provided FIBI scale to a 0-100% scale. For each watershed, the average of the scores was calculated as the overall watershed score.  

Benthic Index of Biotic Integrity (BIBI)

Background: Benthic Macroinvertebrate health (integrity) is indicative of overall stream health. The Benthic Index of Biotic Integrity (FIBI) is a well-established method of assessing the health of benthic stream communities, and has been adapted for various environments and regions (Thomas et al. 2013). 

Data: Data were obtained from various sources. Many organizations and groups monitor BIBI at various locations. Some locations are monitored frequently over time, and others less frequently. For each watershed, BIBI data were obtained from multiple sampling locations. For each sampling location with data, only data from the most recent year sampled were used. 

Rock Creek data were provided by NatureForward Water Quality Monitoring Program, which scores BIBI on a scale of 0-23, where 0 is the poorest score and 23 is the best. Sligo Creek data were also obtained from the NatureForward Water Quality Monitoring Program. Accotink Creek data were provided by the Friends of Accotink Creek Macro-invertebrate Survey, which scores BIBI on a scale of 0-12, with 0 indicating very poor condition and 12 indicating excellent condition.  

Threshold and scoring: The BIBI is a scaled score of benthic macroinvertebrate health, so no set threshold was used. BIBI scores were calculated from the appropriate BIBI scale to a 0-100% scale, setting the lowest score possible equal to 0% and the best score to 100%. For each watershed, the average of the scores was calculated as the overall watershed score.  

Water Chemistry

Background: Many aspects of water chemistry can be used to assess overall water quality. Data for most of our watershed were available for the following indicators: temperature, pH, Dissolved Oxygen, and Specific Conductivity. Each of these has implications for stream health and is important to animals, plants, and algae that make up a stream community. 

Data: Data were obtained from various sources. Accotink Creek data were obtained from USGS Virginia Water Science Center. Sligo Creek data were provided by Montgomery County. Rock Creek data were available from the National Park Service. 

Threshold and scoring: For each stream, multiple measures of dissolved oxygen, pH, water temperature, and specific conductivity were available. For Accotink Creek, two samples per month throughout the year were available for each parameter. For Sligo Creek, year-round data were not available from 2022, therefore the two measures taken in 2022, along with data from the previous 2 years of sampling, 2016 and 2014, were included in analyses. For Rock Creek, year-round data from 2018 were available for each parameter. Each measure of each parameter was scored following the protocols below. Then, for each watershed, the average score for each parameter was calculated. The mean of the average parameter scores for all four parameters was taken as the overall score for each watershed.  

The reported values for each site across each dataset were averaged for each indicator. The average reported value for each indicator was then scored as described below. 

pH: The EPA considers the optimal pH range for most aquatic life forms is 6.5-8.5 (EPA 1). Values in this range scored 100%. Values above 9.6 and below 5 do not support healthy biological communities (Radke 2006, Lenntech 2013). Therefore, values between 8.5 and 9.6 were rescaled along a 100 to 0% scale. Similarly, values between 6.5 and 5 were rescaled along a 100 to 0% scale. Values above 9.6 and below 5 scored 0%. 

Dissolved Oxygen: The National Park Service Natural Resource Condition Assessments consider 6 mg/L to be “very good” in terms of stream quality. This is the most conservative “minimum required DO concentration to support fish” cited in Maryland, Washington DC, and Virginia water quality regulations (Pieper et al 2012). This value was the threshold for a 100% score. The EPA considers “DO levels below 3 milligrams per liter (mg/L)” to be “of concern”, so DO concentrations at or below 3 mg/L scored 0% (EPA 2). Values between 3 and 6 mg/L were scored by rescaling their values along a 0 to 100% scale.  

Temperature: Temperature scoring follows temperatures ideal for trout, a fish that does or has historically inhabited all of the watersheds under consideration. The preferred temperature range of brook trout is 14-19 ^oC, therefore temperatures of 19^oC or lower was the threshold for a score of 100% (Kratzer and Warren 2013). Temperatures at or above 30.8 ^oC received a score of 0%, as this temperature is considered to be lethal to trout (Wehrly et al 2011). Temperatures of 25.3^oC received a score of 50%. This is the threshold temperature at which half of trout fry can survive^{,  (Fry et} al 1946, Wehrly et al 2011). Temperatures between 19 and 25.3^oC were scored by rescaling their values along a 100 to 50% scale. Temperatures between 25.3 and 30.8^oC were scored by rescaling their values along a 50% and 0% scale. 

Specific Conductivity: A minimum specific conductivity of 171 μS/cm is considered ideal for fish health, so Specific Conductivity values at or higher than this scored 100% (Weaver and Nortrup 2016). Specific Conductivities less than or equal to 230 μS/cm have been shown to harm biotic life, and therefore values at or above this scored 0% (Morgan et al 2012). Values in between 171 and 230 μS/cm were rescaled from 100 to 0%.