Inputs and selected outputs used to assess a stream restoration in Flatlick Branch, Fairfax County, Virginia from water year 2008 through 2024

Metrics describing watershed hydrology, stream hydraulics, stream geomorphology, physicochemical processes, and aquatic biology were used to assess a 1.72 km reach of stream above the U.S. Geological Surveys (USGS) Flatlick Branch (station ID 01656903) monitoring station restored using natural channel design. The monitoring station has been continually operated since the start of water year 2008 (water years begin October 1, end September 30th, and are named for the year in which they end). The stream restoration was constructed and completed in water year 2018, and the assessment uses a pre- versus post-restoration study design. The study period spans water year 2008 through 2024. Hydrologic metrics derived from data retrieved from the National Water Information System are provided. Hydraulic metrics are supported by semi-annual cross-sectional surveys collected by USGS staff following standard procedures (Noll and Rylund, 2020). Nutrient and suspended sediment concentrations and loads were computed using a surrogate (multiple-linear regression) approach with lab analyzed nitrogen, phosphorus, and suspended sediment samples as the response variable and basic water-quality parameters (e.g. turbidity, specific conductance, water temperature, pH), streamflow, a baseflow separation Boolean term, and time and seasonal terms as predictor (surrogate) variables. Load results represent the mass of nitrogen, phosphorus, and suspended sediment exported from the watershed. Calibration data, high-frequency surrogate timeseries, and model coefficients, diagnostics, and residuals are provided for each model. High-frequency estimates of concentration and load are provided for all nutrient and sediment constituents. Fairfax County collected benthic macroinvertebrate and habitat data annually, and fish data triennially. These data were used to compute numerous geomorphological and biological metrics and explore patterns and trends before and after the stream restoration. This data release contains seventeen comma-delimited (.csv) files with corresponding data dictionary files (.csv), one text (.txt) file with corresponding data dictionary file (.csv), and one R script. • AEP.csv contains annual exceedance probabilities for peak streamflows derived with the USGS program PeakFQ version 7.2. • Benthic_macroinvertebrates.csv contains raw, field collected benthic macroinvertebrate counts classified by taxonomic hierarchy. Data were collected by Fairfax County. • Benthic_scores.csv contains benthic macroinvertebrate annual scores for 31 metrics describing assemblage richness and composition, and the Fairfax County index of biotic integrity (IBI). • Cross-sections_annual.csv contains annual cross-sectional survey data collected at the monitored cross-section at Flatlick Branch (USGS Station ID 01656903). • Cross-sections_average.csv contains two cross-sections. Each an average of channel cross-sections in the pre-restoration (2008-2017) or post-restoration (2019-2024) periods. • Depth.csv contains an estimate of channel depth based on data collected during manual streamflow measurements from water year 2008 through 2024. • Fish.csv contains fish survey data collected by Fairfax County triennially at Flatlick Branch and Frog Branch. • Floodplain_innundation_events.csv contains gage height data used to quantify the number and duration of events when streamflows exceeded the left bank elevation, causing floodplain inundation. • Floodplain_innundation_event_duration.csv contains the duration, in hours, of each event when streamflows exceeded the left bank elevation, causing floodplain inundation. • Floodplain_innundation_annual_duration.csv contains the total duration, in hours, of each water year when streamflows exceeded the left bank elevation, causing floodplain inundation. • Habitat.csv contains habitat metrics collected by Fairfax County annually at Flatlick Branch and Frog Branch. Habitat assessments are conducted in the late summer or early fall along the same 100-meter (m) reach as the biological assessments, using EPA’s RBP adapted for use within the county. • Load_calibration.csv contains all data used in the calibration of surrogate regression models for the computation of suspended sediment, total nitrogen, total phosphorus, nitrate plus nitrite, orthophosphate, total dissolved nitrogen, total dissolved phosphorus, total particulate nitrogen, total particulate phosphorus, total Kjeldahl nitrogen, and dissolved total Kjeldahl nitrogen concentration and load. • Load_surrogates.csv contains 15-minute interval measurements of streamflow, water-quality parameters, and a Boolean baseflow separation identifier “BASE.” These data were used to compute 15-minute interval measurements of suspended sediment, total nitrogen, total phosphorus, nitrate plus nitrite, orthophosphate, total dissolved nitrogen, total dissolved phosphorus, total particulate nitrogen, total particulate phosphorus, total Kjeldahl nitrogen, and dissolved total Kjeldahl nitrogen concentration and load using surrogate regression models, which were calibrated with the data provided in load_calibration.csv. • Model_residuals.csv contains model residuals for each model. • Model_diagnostics.csv contains diagnostics for each model. • Surrogate_models.csv contains the coefficients for each model. • UV_load_conc.txt contains 15-minute unit value predictions of suspended sediment, total nitrogen, total phosphorus, nitrate plus nitrite, orthophosphate, total dissolved nitrogen, total dissolved phosphorus, total particulate nitrogen, total particulate phosphorus, total Kjeldahl nitrogen, and dissolved total Kjeldahl nitrogen concentration and load from the beginning of water year (October 1 - September 30) 2008 through 2024 at the Flatlick Branch (USGS Station ID 01656903) monitoring stations in Fairfax County, Virginia. • rLOADEST.code.R contains R code to generate 15-minute interval estimates of load at the Flatlick Branch (USGS Station ID 01656903) monitoring station for suspended sediment, total nitrogen, total phosphorus, nitrate plus nitrite, orthophosphate, total dissolved nitrogen, total dissolved phosphorus, total particulate nitrogen, total particulate phosphorus, total Kjeldahl nitrogen, and dissolved total Kjeldahl nitrogen. For each file (excluding the r code) entity and attributes are described in data.dictionary.csv that shares the same name.

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