The impact of changing landscape connectivity on sedimentation in Karoo farm dams, South Africa: the case of breached dams

Our research in the Eastern Cape over the last 10 years has demonstrated that changing landscape connectivity, often linked to badlands or gullies (locally known as dongas), has been a major feature in explaining changes in sediment yield and provenance through time in the study sites already reported in the published literature. Here we tackle a new issue in connectivity by focusing on the potential impact of dam breaching on downstream sediment dynamics. Our basic hypothesis is that the breaching of farm dams, usually by major floods, and subsequent failure to repair them leads to two new sources of sediment for the downstream catchment. First, sediment stored within the breached dam will be evacuated by vertical and lateral incision as stream (donga) long profiles re-adjust to renewed (pre-dam) base level conditions. Secondly, sediment from the upstream catchment that would have otherwise accumulated in the dam is now reconnected to the downstream donga and adds to the sediment being evacuated from the farm dam. If the combined effect of these two factors is significant, we would expect to see an increase in sedimentation rates in any downstream farm dam that remained un-breached. Our test of this hypothesis utilizes two farm dams in the Good Hope valley located in the Sneeuberg Uplands. The upstream dam (Dam 53; catchment area 2.53 km2) breached during the major Eastern Cape floods of 1974 and has remained un-repaired since this time. We estimate that over 4,500 m3 of sediment has been eroded from the dam and delivered to the downstream Dam 37 since this time. The downstream dam was constructed around the 1950s and was isolated from the catchment area upstream of dam 53 until 1974. We use a combination of 210Pb and 137Cs dating and a range of tracer properties to establish the impact of the breach in Dam 53 on downstream sediment delivery.