State of the Streams 2024

Methods – Climate

Tree Canopy 

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. 

Air Quality

Background: Air quality is impacted by a number of factors, not only the presence or number of trees. Carbon emissions caused by vehicle traffic is of particular concern in urban areas, and mitigation of air quality is difficult to achieve. Nevertheless, it is important to consider this indicator when thinking about climate.

The Air Quality Index (AQI) is a scientific, commonly used scale to measure and report on the amount of pollutants in the air. Levels of five pollutants can be measured using the AQI: Ozone, Particulate Matter (PM2.5 or PM10), Carbon Monoxide, Sulfur Dioxide, and Nitrogen Dioxide. Levels of one of these, Particulate Matter, were used to score this indicator. Particulate matter is defined by the National Park Service as “fine particles less than 2.5μm diameter” that “are emitted as smoke from power plants, gasoline and diesel engines, wood combustion, steel mills, and forest fires”. Because particulate matter is harmful to human health, it is regulated by the EPA, which sets maximum allowable particulate matter levels. 

Data: Data were obtained from the PurpleAir database. PurpleAir manufactures air quality monitors that community scientists install in various locations around the country. These monitor PM2.5 in the air and report the data to the PurpleAir database. The data are provided as daily AQI scores ranging from 0 to 500, with values corresponding to air quality that is “Good”, “Moderate”, “Unhealthy to Sensitive Groups”, “Unhealthy”, “Very Unhealthy”, and “Hazardous” (EPA Outdoor Air Quality). Daily PM2.5 AQI scores were downloaded for the most recent 365 days for which data were available from six monitors in the Rock Creek Watershed (two at each of the following sites: Mount Pleasant—17th and Irving, Georgetown Park, and BCC), six in the Sligo Creek watershed (two at each of the following sites: Takoma Park, Fleetwood, and Ethan Allen Ave), and five in the Accotink watershed (two at each of the following sites: Springfield, VA and “Home”, and one at “Mirror Pond”). 

Threshold and scoring: Each daily AQI value from each monitor at each site was rescaled to assign scores from 0-100% as shown in the table below. An average score was then calculated for each monitor. For sites with multiple monitors, an average site score was calculated. Because not all sites had the same number of daily scores (sometimes monitors were offline and did not report), the average score for each site was weighted by on how many daily scores there were, then site scores were aggregated into a final score for each watershed.  


Stream (Riparian) Buffer

Background: As climate change continues, stream temperatures are increasing to levels harmful to aquatic life (Nelson and Palmer 2007). When a riparian area, or area along a stream, is vegetated, stream temperatures are able to maintain microclimates and cool water temperatures. In fact, the cooling impact of stream buffers can be so great that it benefits fish communities (Justice et al 2017). Vegetated riparian area, also called “riparian buffers”, also help prevent erosion.

Many nonprofits and government agencies recommend maintaining a vegetated stream buffer of at least 100 feet (NY Department of Environmental Conservation, Virginia DCR 2007, Maryland DNR), and legislation has been implemented requiring such a buffer in some states and counties. The Chesapeake Bay Agreement is working toward a goal of replanting riparian areas until 70% of them are vegetated. Maintaining a 70% vegetated area along every stream is not possible; NOAA has set a target of 40% vegetative cover for urban areas (NOAA riparian). 

Data: GIS data (USGS National Hydrography Dataset) were used to calculate the percent of riparian buffer that is forested in each watershed.  

Threshold and scoring: The percent of riparian area that is forested was scored along a scale so that if 70% of riparian area is vegetated, meeting the Chesapeake Bay Agreement goal, the score is 100%. If 40% is vegetated, meeting the minimum recommended vegetative cover for urban areas, the score is 50%. Cover between 40% and 70% of riparian area is scored along a scale of 50 to 100%. Coverage less than 40% is rescaled so that 40% cover receives a 50% score and 0% cover receives a score of 0%. 


Background: Wetlands mitigate many of the effects of climate change, while also being threatened by climate change. They provide habitat, reduce flooding by holding excess water, and store greenhouse gases (Finlayson 2018, Kayranli et al 2010, Endter-Wada et al 2020). When wetlands are lost, they emit greenhouse gases.   

Data: 30x30m resolution GIS data from the 2019 National Land Cover Database were used to calculate the total change in wetland area within each watershed over the 15 year period from 2004 to 2019.  

Threshold and scoring: This indicator was scored as pass/fail. If any wetland area was lost over the 15 year period, a watershed scored 0%. If wetland area was maintained or increased, the watershed scored 100%. This scoring scheme is also used in the Maryland Coastal Adaptation Report Card (Laumann et al 2022).