LANDSCAPE EVOLUTION, HISTORICAL ECO-GEOMORPHOLOGY AND RIVERINE PROCESS DOMAINS: DEFINING A “REFERENCE CONDITION” FOR RIVER RESTORATION AND MANAGEMENT IN THE PUGET SOUND BASIN

Identifying a “reference condition” for large floodplain rivers—and how to achieve it—is often made difficult by the homogenizing and simplifying effects of anthropogenic change. As a consequence, river management and restoration often rely on assumptions that often prove incorrect. In the geologically dynamic Puget Sound basin, diverse processes shape rivers, and humans have extensively influenced them since Euro-American settlement (~1850); both complicate identifying a reference condition. To determine riverine reference conditions in the Puget Lowland we examined physical processes and forms at a hierarchy of scales—from annual to Holocene, and channel-width to region—and how they create a template for, and are in turn modified by, ecosystems. We used multiple, cross-referencing approaches—field studies, archival sources, natural proxy data, and high-resolution digital elevation models—to understand Holocene landscape evolution, create process models, and map pre-settlement landscape. We found the legacy of Pleistocene glaciation to be the dominant geologic influence; it creates two divergent disequilibrium fluvial responses within the same physiographic province that generate fundamentally different riverine dynamics, landforms, and ecosystems. Holocene lahars modify this template at the watershed scale, co-seismic uplift and subsidence at the reach scale, and dynamics between forests, rivers, and fluvial wood at the reach and channel scale. A regional model for fluvial landscapes in context of a “riverine process domain” concept (river valleys or reaches having similar or analogous processes creating similar dynamics, landforms, and ecosystems) and GIS mapping of pre-settlement landscape provide tools to guide management and restoration. Absent such a multi-scale, spatially explicit reconstruction of pre-impact riverine processes and landscapes, it is possible to miss important historical processes, landforms, and heterogeneities no longer present or obvious in the landscape, leading to a limited or distorted understanding of riverine function and structure.