Riparian areas (land along river banks) are a natural place for human settlement given the richness of floodplain soils for farming, their proximity to freshwater and the opportunity for transportation and commerce, as seen in antiquity with the settlement of the Nile River delta and more recently with the collection of cities dotting the shores of the Mississippi River and its delta.
Recent reviews of salmonid habitat restoration programs have recommended that managers emphasize strategies that restore natural habitat-forming processes, such as restoring riparian vegetation, over placement of instream structures. In addition to the direct benefits of shading and providing a source for large woody debris (LWD), riparian restoration is often implemented to improve channel morphology for purposes of restoring fish habitat. However, multiple studies provide equivocal evidence that restored vegetation can lead to improved channel form within a period of years to decades. In this study, we examined the effectiveness of riparian restoration for improving channel morphology and fish habitat in four hardwood-dominated streams in Mendocino County, California. These streams support populations of steelhead Oncorhynchus mykiss and contain reaches with riparian corridors that were restored through exclusionary fencing implemented 10-20 years earlier. We compared channel morphology, LWD, and late-summer water temperature between the restored exclosure reaches and geomorphically similar control reaches within the same properties. Channels within exclosures were significantly narrower and had greater heterogeneity in long profile elevation than control reaches. Frequencies of LWD and debris jams were considerably greater in exclosure reaches than control reaches and were comparable to values from similar streams with mature forests. Late-summer water temperature in exclosures was within the acceptable range for steelhead, whereas water temperature in control reaches was warmer and potentially detrimental to steelhead. Riparian restoration in exclosures has resulted in quantitatively improved habitat characteristics and qualitatively different channel morphologies as compared with control reaches. The ability of vegetation to improve channel morphology in this region is probably due to frequent overbank flooding and high sediment loads. Through a comparative analysis of the cost and performance of exclusionary fencing versus those of instream structures, we propose that riparian restoration can produce instream salmonid habitat benefits that are more comprehensive, sustainable, and cost-effective than the benefits generated by instream structures.
We describe a process and methodology for quantifying the extent of a type of historically prevalent, but now relatively rare, ecologically-valuable floodplains in the Sacramento lowland river system: frequently-activated floodplains. We define a specific metric the “Floodplain Activation Flow” (FAF), which is the smallest flood pulse event that initiates substantial beneficial ecological processes when associated with floodplain inundation. The “Activated Floodplain” connected to the river is then determined by comparison of FAF stage with floodplain topography. This provides a simple definition of floodplain that can be used as a planning, goal setting, monitoring, and design tool by resource managers since the FAF event is the smallest flood and corresponding floodplain area with ecological functionality—and is necessarily also inundated in larger flood events, providing additional ecological functions. For the Sacramento River we selected a FAF definition to be the river stage that occurs in two out of three years for at least seven days in the mid-March to mid-May period and "Activated Floodplains" to be those lands inundated at that stage. We analyzed Activated Floodplain area for four representative reaches along the lower Sacramento River and the Yolo Bypass using stream gauge data. Three of the most significant conclusions are described: (1) The area of active functional floodplain is likely to be less than commonly assumed based on extent of riparian vegetation; (2) Levee setbacks may not increase the extent of this type of ecologically-productive floodplain without either hydrologic or topographic changes; (3) Within the Yolo Bypass, controlled releases through the Fremont Weir could maximize the benefits associated with Activated Floodplain without major reservoir re-operation or grading. This approach identifies a significant opportunity to integrate floodplain restoration with flood management by establishing a FAF stage metric as an engineering design criterion alongside the commonly-used 100-year flood stage for flood hazard reduction.