PA Aquatic Resource Restoration Conference 2026: Past, Present and Future

Little Arnot Run, a 9.51km3 watershed in the Allegheny National Forest, Pennsylvania, is typical of forested headwater streams in the Allegheny Plateau region — narrow alluvial valleys surrounded by steep valley sides that were clearcut during the 19th-century, with managed silviculture and oil and natural gas operations continuing today. The streams remain relatively straight and incised, confined by relic railroad berms, and disconnected from abandoned side channels and floodplain. 

Beginning October 2019, detailed geomorphic surveys were combined with hydrologic data collected at a network of 1 weather station, 4 stream gages. and 16 piezometers installed along a 2km reach of the valley floor, show hyporheic exchange potential (HEP) to be approximately 1 x 10-4 , suggesting excellent potential for valley-wide groundwater storage and hyporheic uplift. 

Lidar and field survey data suggested a pre-logging geomorphic grade line (GGL) of 0.012 could be achieved if large wood debris (LWD) and sediments from relic railroad berms were returned to the channel Prior to restoration, little to no hyporheic exchange occurred during summer and fall months, with riffles in the middle and lower reaches becoming completely dry. 

 Following restoration in August 2021, the lateral extent of hyporheic zone remained greatest during wet winter and spring months, but increased to as much as 30 m from the channel. Hyporheic flux rates from the stream to the adjacent alluvial aquifer remained high 0.23 m3 s 1) during the spring months, when water levels in the stream are high. During the driest period of the year (August - October), water levels are 0.2m higher across the valley and hyporheic exchange rates increased from 0 to 0.19m3 s-1 Because of the bedrock-alluvium architecture, groundwater gradients continue to be greatest down the axis of the valley and down-valley flow estimated to be 3 to 5 times greater than lateral flow rates. However, increased lateral hyporheic exchange during the hot, dry summer and fall months has consistently decreased stream temperatures during the summer by 2-4°C for the past five years. Side channels, now reconnected and flowing year round, provide refugia for native brook trout during spring floods and spawning habitat in the fall.