S2 Data.

Prior to mass deforestation of uplands and drainage efforts in lowlands, floodplain wetlands were abundant in river valleys. Deforestation of steep slopes and the associated accumulation of alluvial sediment in valley bottoms have left floodplains much drier and largely disconnected from their adjacent rivers, restricting the ecosystem and societal benefits floodplains can provide. By exporting alluvial sediments from floodplains, long buried wetlands can be restored with improved connectivity to stream networks. Such floodplain wetland restoration efforts can provide benefits at orders-of-magnitude lower costs than conventional stormwater control measures (SCMs) by minimizing pipes and other hardened infrastructure. Particularly in urban and suburban watersheds, floodplains are often one of the last open areas to locate SCMs that have the potential to intercept large volumes of runoff. They can also be coupled with recreational opportunities or urban canopy restoration programs, expanding the societal benefits of and opportunities to fund floodplain wetland restoration. This paper presents insights from planning, design, modeling, and construction of eight floodplain wetland restoration projects. The eight projects were compared using success criteria that spanned two themes: 1) maximizing the benefits that the floodplain wetland can provide and 2) maximizing the economic efficiency and durability of the project. As a part of the comparison, we note characteristics for identifying good sites for floodplain wetlands, design optimization strategies for various project goals (e.g., nutrient reductions vs. offloading excessively erosive streamflow vs. flood reduction), and construction sequencing approaches for efficiency and cost minimization. Removing alluvium from floodplains can provide expanded flood storage and lowered flood elevations, restored off-channel habitat for fish/birds, improved water quality, reduced erosion/biotic disturbance for mussels/macroinvertebrates, and, in some cases, the economic benefits of an abundant source of high-quality topsoil for farmers, landscapers, and developers.