This technical memorandum describes an assessment of future climate impacts to riparian ecosystem conditions and functions in the Petaluma River watershed and adjacent drainages (the Petaluma Operational Landscape Unit). Four bias-corrected, downscaled global climate models (GCMs) with climate predictions through the year 2100 were analyzed and used for hydrological modeling. The analysis focused on three time periods - early century (2016-2045), mid century (2040-2069), and late century (2070-2099). Downscaled GCMs predict a 1-2°C temperature increase by mid century and 2-4°C increase by late century. Average annual rainfall differs among models and is expected to be neutral or increase compared to current levels. High-flow events and sediment load are projected to remain fairly consistent through the early and mid century, and to increase in late century. Low flows are projected to decrease in the early-to-mid and late century periods, impacting intermittent streams within the watershed. Groundwater tables may decline as a result of reduced precipitation, longer dry seasons, and prolonged and/or severe droughts.

A conceptual model of drivers and responses was developed to provide a framework for understanding and evaluating riparian responses to climate, physical process, and human drivers, as well as the resulting impacts to key riparian ecosystem functions including channel shading, bank stabilization, fish and wildlife habitat, allochthonous inputs, and runoff filtration. Modeled projections of flow and sediment were evaluated with this framework for different landscape units (lowland valley, Northeast and Southwest-facing headwater and foothill settings) which were established based on distinct physical characteristics including geology, aspect, and precipitation. Combined, the downscaled climate model output, conceptual model, and landscape units informed a spatially explicit approach to identify likely changes to riparian ecosystems, functions, and their proximate drivers.

Declining low flows projected in both early-mid and late century across all four models may result in a loss of groundwater connection in some settings, with contraction of riparian vegetation forested width and local declines in phreatophytic or groundwater-dependent vegetation. By late century, increased high flows during regular flood events may result in local increases in large woody debris and bank erosion. The framework developed for this effort can be employed when planning management actions that address or account for potential climate futures. Understanding likely climate change impacts to riparian vegetation characteristics such as physical habitat complexity, community composition, and forest width are critical to understanding changes in riparian ecosystem functions.