THE GEOMORPHIC LEGACY OF RANGELAND WATER AND EROSION CONTROL STRUCTURES

Control over water supply and distribution is critical in the western US where manipulating surface runoff often serves the dual purpose of erosion control. However, little is known of the geomorphic impacts and legacy effects of rangeland water manipulation infrastructure, such as earthen berms, water control gates, and stock tanks, especially if not maintained. This study investigated the geomorphic impacts of legacy structures in a semiarid rangeland in the southwestern USA that is responding to both regional channel incision that was initiated over a century ago, and a more recent land use change that involved cattle removal and abandonment of structures. We inventoried, mapped, and assessed the structural condition of remnant structures using aerial imagery and lidar data. We identified headcut initiation, scour, and channel incision associated with compromised stock tanks, lateral channel berms, concrete water control structures, and floodplain water spreader berms as threats to floodplains and associated habitat. Based on analysis of aerial imagery, of 27 identified lateral channel berms, 48% have been breached and 15% have been flanked, 18% of 218 shorter water spreader berms have been breached and 17% have been flanked. Loss of historical perspective often accompanies change in land ownership, however knowledge of past landscape manipulation is critical because both current land management and future land use options are constrained by inherited land use legacy effects.