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Patterns of channel change on Chilliwack River, British Columbia Ham, Darren Gary


This study investigates changes to channel morphology along Chilliwack River in southwest British Columbia, and relates these changes to the transport of bed material. The channel was mapped using a stereoplotter from a sequence of historical aerial photography between 1952 and 1991. Maps for selected years were overlaid, then analyzed with a GIS. Erosion and deposition volumes of bed material were determined by multiplying measured planimetric changes by the varying depth of bed material along the river. A sediment budget framework was used to estimate bed material transport rates from these net changes in stored sediment volumes. The transport rate of bed material for Chilliwack River is estimated as 55,000±10,000 m3/yr for the period 1983 to 1991. Instability along lower channel reaches has increased over the 40 year period of study due to an increase in both the magnitude and frequency of large floods. As a consequence, channel width, bank erosion rates and the volume of sediment transported past Vedder Crossing became increasingly large. Between 1952 and 1975, Chilliwack River was in a transient state of equilibrium as the channel continued to recover from a sequence of large floods in previous decades. Large floods in 1975 and 1980 caused significant bank erosion along lower reaches, which increased the amount of sediment available for transport. Extreme floods in 1989 and 1990 caused further erosion, and in fact, were large enough to alter the pre-flood channel regime. However, in the absence of further large floods, the channel should recover from these events in 10 to 20 years. Significant morphologic change on Chilliwack River occurs roughly once every 5 years, when peak flows exceed 500 m3/s. These floods are sufficient to erode channel banks, where the dominant supply of mobile bed material is stored. Aggradation occurs in the short term as more sediment is introduced to the active channel zone than can be removed by subsequent smaller flows. However, as flows as small as 250 m3/s (which occur several times per year, on average) are capable of mobilizing bed material, there is net degradation over the longer term. As the length between survey dates used in this study averaged 10 years, some information on bed material transport between dates is lost. Nonetheless, this study has demonstrated that considerable information on historic channel change can be obtained form aerial photographs.

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