| Summary: | The purpose of this report was to determine whether the Avon Dam, situated in the River Avon’s headwaters in South Devon, has had a detrimental impact on the quality of salmonid spawning gravels downstream of the dam. The report also aimed to examine the possibility of success of a gravel augmentation scheme to replenish and improve the quantity and quality of spawning sized sediment. To achieve this, classifications of in-situ gravels at 8 sites across the upper Avon and a sample of augmented gravel were taken in order to determine changes in size, using Wolman counts, and shape, using Zingg classifications. Cross sectional measurements were also taken and used to calculate incipient motion values for the sites. Finally, mapping of the river channel allowed an appreciation of any existing sedimentary structures that existed in the channel. The results showed that there had been a coarsening of gravels downstream of the dam with the sediments pushed above the recommended ideal spawning size of Atlantic salmon. It was also determined, due to the high potential for transport shown by the incipient motion calculations, that any augmented gravels placed into the channel should be easily entrained and the presence of certain sedimentary structures suggests that the augmented gravels should remain in the desired reach of the river long enough to form stable structures consisting of good quality spawning substrate. In order to confirm these predictions direct monitoring of the augmented sediments, using RFID tags and impact plates, could be used to map the locations that certain gravels are deposited and understand when they are mobilised.
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