A century of channel change caused by flow augmentation on Sixth Water Creek and Diamond Fork River, Utah, USA.

The Diamond Fork River in central Utah, USA experienced extreme flow augmentation via transbasin flows. Beginning in 1916, irrigation water was delivered through a tributary, Sixth Water Creek, with daily summer flows regularly exceeding a 500‐year flood at the point of introduction. Flows were dramatically reduced by management action in 2004 but with mandated minimum flows. We examined the geomorphic response of Sixth Water and Diamond Fork using aerial imagery, lidar, topographic cross sections, and sediment transport measurements. River channel response varied with valley confinement and with position in the watershed, which determined the magnitude of augmented flow relative to natural floods and the amount of sediment supply. Confined, steep sections of Sixth Water incised many meters into bedrock, whereas partially confined and unconfined sections developed a braided form even under conditions of general incision. With the removal of flow augmentation, smaller natural floods on Sixth Water are unable to transport bed material and the channel remains static. On the alluvial lower Diamond Fork, a lower slope, upstream sediment supply, and larger natural floods produced a dynamic shifting channel that widened in response to natural floods. After flow augmentation, a coarsened bed is partially mobile and channel narrowing appears to be limited by artificial baseflow, which prevent vegetation establishment in the channel. [ABSTRACT FROM AUTHOR]