LB 5-49 - Silver Maple trees as a record of altered flow regime of the Allegheny River

The Sustainable Rivers Program aims to restore components of the natural flow regime in regulated rivers across the US. In partnership with the Nature Conservancy, the Pittsburgh District of the US Army Corp of Engineers is implementing environmental flows to work towards this restoration goal through scheduled pulse events released from the Kinzua Dam.Floodplains are the ecotone between aquatic river systems and the adjacent terrestrial ecosystems. Inundation of floodplains is essential to the ecology of this system, where the exchange of water, sediment, nutrients, and biota between the aquatic and terrestrial environments occurs during high river flows. During these high flow events, soil moisture increases laterally into the floodplain. Acer saccharinum and Platanus occidentalis are prominent native trees along the Allegheny River and are adapted to soil moisture conditions related to the natural flow regime. Kinzua dam operations on the Allegheny River, however, have decreased the frequency and intensity of high flows. To quantify the disruption of floodplain connection, we conducted a survey of tree abundance and age and monitored soil moisture along a transect perpendicular to the Allegheny River downstream of the Kinzua dam at West Hickory, PA. To understand the response of riparian trees to historical river flow trends, we conducted tree ring analyses of the floodplain trees.We found that soil moisture was positively influenced by river discharge. Under typical March flow conditions, soil moisture within 1 m of the riverbank is controlled by discharge, but beyond 1 m is controlled by precipitation. During an experimental high flow event, soil moisture response extended to ~6 m from the riverbank, indicating that high flow events increase floodplain connectivity. The tree survey revealed that tree abundance and relative age show no relationship with distance from the riverbank. This indicates that in recent history, high flow events are not strong or frequent enough to adequately disturb the near-bank riparian zone. Tree ring analysis revealed lower growth rates in rings set after the installation of the dam, reflecting the dam induced changes in the hydrology of the river. Analysis of historical flow regime revealed that maximum flows have decreased, while minimum flows have increased since the installation of the dam, supporting the results of the tree ring analysis. Together, these results suggest that the implementation of environmental flows can re-establish connectivity between the floodplain and the river in order to healthy, active floodplains and riparian ecosystems.