Human-modified ponded features mapped within the Willamette River floodplain, Oregon, in 2018

The floodplains of the Willamette River and its major tributaries in western Oregon comprise a diverse array of natural and human-modified features that collectively support a wide range of habitats used by native fish and other wildlife. Like other large alluvial rivers, the Willamette River floodplain has many water bodies disconnected from the main river channel throughout most of the year. These features consist of relict channel features abandoned through channel migration or avulsion, as well as human-modified features such as irrigation ponds and gravel pits where rock and aggregate were formerly mined for commercial purposes. Since 2008, large-scale restoration programs have been implemented along the Willamette River and its major tributaries to address historical losses of floodplain habitats caused by dam construction, bank stabilization, and various land-use changes (Keith and others, 2022). One restoration strategy to improve habitats used by native fish has been to modify former gravel pits. Gravel pit restoration has included a variety of site-specific strategies such as improving hydraulic connectivity with nearby river channels, modifying pond topography to reduce fish stranding, and planting native vegetation (Keith and others, 2022). Because gravel pit restoration was identified as a key focus for floodplain restoration (Anchor Habitats Working Group [AHWG], 2015), stakeholders in the Willamette River Basin identified a need for a map showing the distribution of floodplain ponds that may have been human-modified and that may be candidates for future restoration activities. Hence, ponded features were mapped from 2018 aerial photographs (Oregon State Imagery Program [OSIP], 2018) to document the locations and spatial distribution of sites where gravel extraction or other human modifications may have resulted in pond topography or characteristics that differ from disconnected floodplain channels likely created through natural fluvial processes (such as abandoned oxbow channels). No attempt was made to confirm the type or extensiveness of human activities that have occurred within the mapped ponded features. This mapping dataset, together with other mapping resources, is intended to be a useful starting point for considering the locations, sizes, and distribution of human-modified ponded features (including former gravel pits) in the Willamette River floodplain and identifying potential priorities for future restoration. The 2-year recurrence interval floodplain (mapping by River Design Group, Inc., 2012), for example, is often used to plan and prioritize floodplain restoration and conservation efforts because the boundary is clearly defined and maps are publicly available (Keith and others, 2022). The mapping of human-modified ponded features encompassed areas within the Holocene (or geomorphic) floodplain of the alluvial reaches of the Willamette River (from Newberg to Eugene).