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Physical Processes
Aerial photographs taken in the 1940s show the mainstem Napa River valley fully developed into agriculture. At this time, the mainstem Napa River, above the city of Napa was a low gradient, gravel-bedded stream exhibiting bar-pool morphology, with point bars, mid-channel or island bars and multiple channels in the unconfined reaches. These reaches were bordered by floodplains that appeared to be inundated annually with well-established vegetation. There were well-developed wetlands located in the transitional areas between alluvial fans and the valley floor, and on the floodplains. In confined reaches, the Napa River was a single-thread channel with the extent of the active floodplain generally constrained by coarse-textured, erosion resistant tributary alluvial fans.
Prior to major anthropogenic disturbances in the basin, the Napa River would have had numerous side channels that provided backwater rearing habitat for salmonids. The mainstem channel would have been connected to its floodplain in most locations, with the floodplain inundated during several storms per year. In contrast, 1998 aerial photographs depict a simplified riverfloodplain system in which the channel has narrowed, incised, and largely abandoned its former floodplain, resulting in a loss of backwater rearing habitat. Review of channel cross-section records, published reports, and recent field observations indicate that the river has incised at least 4–6 ft (1.2–1.8 m) on average from the mouth of the river to a point upstream of Calistoga, and is currently in the process of active channel incision upstream of Calistoga. Figure 6-1 illustrates some of the changes that occurred between 1940 and 1998 in three reaches. The abandonment of the floodplain and the present-day channel entrenchment are most likely caused by anthropogenic impacts, such as draining and diking of the valley floor, filling of side channels to facilitate development of the floodplain, mainstem channel straightening, mainstem bank stabilization, levee construction, gravel dredging downstream of the City of Napa, gravel bar skimming, loss of bedload supply due to dam construction, and large woody debris (LWD) removal on the mainstem.
These types of alterations of the mainstem river appear to have generally occurred throughout the valley floor, from Calistoga downstream to the City of Napa. Our interpretations of aerial photographs are supported by observations made during surveys of mainstem reaches (approximately 10 miles [16 km], reaches are shown in Map 9), and data from previously published studies. These alterations to the mainstem have affected the quality and abundance of suitable aquatic and riparian habitat for native species. The natural bar-pool morphology evident in the 1940s aerial photos and expected in a wandering river such as the Napa River, with its alternating sequence of pools and riffles, has been converted in many reaches into a series of long run-pools (i.e., long pools that are shallow relative to their length) separated by very small bars. These long run-pools create lake-like habitat for non-native predator fish, increasing the exposure of native salmonids to predation during rearing and outmigration.
Some pool filling by fine sediment was noted in the long run-pool habitats. Channel incision may have increased bed mobilization, which in turn may have increased frequency and intensity of scour of salmonid redds during the winter months (see Section 6.2). Floodplain abandonment has resulted in the loss of side channel, backwater, and slough habitats that would have provided high quality rearing habitat. Throughout most of its length, the mainstem Napa River now has only a narrow band of riparian vegetation.
Stillwater Sciences - Napa River Watershed Limiting Factors Analysis