| Summary: | Salmonids (salmon and trout) are threatened, endangered or at-risk throughout much of their historic range in western North America. However, a lack of detailed information about historic floodplain, riparian and channel condition makes it difficult to disentangle potential causes of population declines (e.g., habitat degradation vs. overfishing). For this research, historic habitat was reconstructed along five salmonid-bearing tributaries of the Columbia River using historic (1949) aerial photographs. Historic airphotos were scanned and orthorectified to create a 0.5 m resolution orthomosaic compatible with a USDA National Agricultural Imagery Program (NAIP) compressed county mosaic of modern (2006) imagery. Floodplain extent, land cover and stream reach type, as well as several types of floodplain and channel habitat important to salmonids, were delineated manually for both time periods. These features were compared between 1949 and 2006, across the five tributaries and at the reach level, using a combination of transition matrices, paired t-tests and ANOVAs. Quantifying these features helped provide historic and modern landscape-level indicators of floodplain, riparian and riverine condition.
The predominance of historic agriculture and contemporary urban expansion have likely reduced important salmonid habitat features in the Wenatchee system. Transitions of island braided channels to straight reaches were associated with significant reach shortening and increased anthropogenic modification to floodplain and riparian areas. When comparing the 1949 and 2006 landscapes, a loss of meandering reaches coincided with a reduction in the types of off-channel habitats associated with lateral channel migration (such as slow/stagnant and dry channels). Increased fragmentation of remaining habitat in highly modified river-floodplains may reduce the accessibility of contemporary floodplain habitat to salmonids. Unavoidable mapping error (due principally to solar glare) likely contributed to overestimating the historic predominance of straight reach types and an underestimate of island braided reaches.
This research provides an example of how anthropogenic activities may have contributed to declines in high quality salmonid habitat in the study area and can guide the prioritization of restoration efforts. The general methodology developed here can be adapted to other river floodplains throughout Canada and the USA where extensive anthropogenic modification may be of management concern.
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