State of the Streams 2024

Methods – Biodiversity & Habitats

Stream Blockages/Fish Passage 

Background: Migratory fishes including river herring are important residents of all three watersheds in this report. These fish must be able to migrate up and downstream to successfully reproduce and grow.  They are prevented from doing so by blockages including dams, but restoration efforts have been focused on removing such barriers. In some areas, these restoration projects have resulted in a return to historic habitat by migratory fishes.

Data: We used the Freshwater Network’s Fish Passage Prioritization Tool to identify barriers to fish migration within each watershed. In addition to mapping barriers, this tool ranks each barrier on a scale from 1 to 20 in terms of how important it is to remove each barrier (in order to enable fish passage) for both diadromous (moving between fresh and saltwater) and resident (remaining in the same water body) fishes. These ranks were used in scoring. 

Threshold and scoring: The threshold for a 100% score for this indicator was that no blockages occur in the watershed (as listed in the Fish Passage Prioritization Tool). We split each watershed into segments based on blockage locations. Each segment was scored by rescaling the blockage impacting it from the 1 to 20 rank along our percent score scale of 0-80%. Segments with no blockages (from the mouth of the watershed to the first blockage) scored 100%. Each segment was then weighted by the percent of the total watershed length it makes up, and the overall watershed score was calculated by adding these together. 


Background: Birds are often used as indicators of ecosystem health (O’Connell et al 2007). The number and type of birds present can indicate habitat quality. Bird biodiversity (the number and types of species present) can be used as a proxy for overall biodiversity, as it has been shown to correlate with the biodiversity of other groups including plants, amphibians, and butterflies (Blair 1999, Katie et al. 2004). This indicator was scored in two different ways, using two methods, one of which applies to bird habitat use and one to biodiversity. The average of the two scores was the overall bird score for each watershed. 

Data: Data were collected from eBird, a database of information collected from citizen scientists across the country (Sullivan et al 2009). Ebird data have been used in various scientific studies of, for example, bird populations (Walker and Taylor 2017, Clark 2017). 

Threshold and scoring: 

Bird habitat use- We identified priority birds using  Audubon’s Priority Birds 2021 lists of birds of conservation concern specific to 1) bird-friendly communities, 2) coasts, 3) water, and 4) working lands. We explored eBird data to see which, if any, species on these lists have ever been recorded in each watershed. This generated a list of priority birds that have historically used habitat along each river/stream. We then determined which birds on each watershed’s list still use habitat in the watershed; if a bird has been recorded along the stream/river in the past five years, it was considered to still use that habitat. The score for bird habitat use was calculated as the percent of each watersheds priority birds that still use that watershed’s habitats. 

Bird biodiversity- Using eBird, we determined the total bird species richness within each watershed in the last five-year period. We compared this number to the species richness from 2006-2010 and calculated the percent by which species richness has declined. The score for this indicator was calculated as 100% minus the percent reduction in species richness. 

Physical Habitat Index (PHI) or Benthic Habitat

Background: Benthic Habitat health scoring was based on the Physical Habitat Index. Physical Habitat Index is an indicator of the physical habitat of a stream. It takes into account important factors such as substrate, water flow, and erosion, all of which impact habitat suitability for various organisms (Thomas et al. 2013, MBSS 2003). 

Data: Data were available from Fairfax County for 17 sites in Accotink Creek. Fairfax County PHI values range from 0 to 200. Data were available from the Maryland Biological Stream Survey (MBSS) for two sites in Rock Creek and one site on Sligo Creek. The most recent data for Rock Creek and Sligo Creek were from 2017 and 2004 respectively, and may not represent current conditions. MBSS calculates PHI values along a scale of 0-20. 

Threshold and scoring: Each PHI value was rescaled to calculate a score so that a PHI value of 0 received a score of 0% and the highest PHI value (200 for Accotink Creek and 20 for Rock Creek and Sligo Creek) scored 100%. 


Invasive Plants

Background: Invasive plant species take up space and resources needed to support a healthy, native plant species community. The percent area covered by invasive species is an indicator of how much space is unavailable for native plant species, and is frequently used as an indicator of ecosystem health (eg O’Loughlin 2021). Recent work shows that the number of invasive species is just as important, if not more so, in determining the potential threat of invasive species to an ecosystem (Wang et al. 2022). 

Data: Data on percent cover by invasive plants was not available, so only the number of invasive species present was used to score this indicator. iNaturalist, a database where citizen scientists can upload sightings of various species, was used to determine how many of the 50 most invasive noxious plants in the region are present in each watershed. 

Threshold and scoring: Because it is unreasonable to expect any area to have no invasive plants, the lowest possible score for this indicator was 50%. The score for each watershed was calculated as 100 minus the number of invasive species reported on iNaturalist.