Suspended Sediment Capture and Buffering by a Tropical Wetland Complex from Satellite Observations

The Magdalena River, Colombia, has the highest sediment yield of any major South American river. This high natural level and recent land cover change in the watershed have exacerbated the sediment load and deposition in the river's lowland floodplains. Excess sediment delivery has led to concerns regarding the ecological integrity of its floodplains and lakes, coral reef burial at the river mouth, and increased dredging needs. The largest floodplain system of the river, the Momposina Depression–a vast wetlandscape formed by > 100  interconnected wetlands, lakes, and floodplains where the Magdalena and Cauca rivers meet, including two Ramsar-designated sites–is accumulating and potentially buffering a large portion of this excess sediment load. However, mechanistic descriptions and seasonal-to-decadal variability of these processes are poorly understood. To fill this gap, we use MODIS and Sentinel-1 imagery at monthly timescales to investigate the spread of turbid water across the system and build a conceptual model for how sediment is captured and remobilized. We find that flooding in the early wet season can have turbidities as large as the highest-discharge periods, but turbidity can vary +/- 40% and flow is generally constrained to the main channels, thus leading to lower consistent floodplain sedimentation delivery. Later in the seasonal flood pulse, overbanking river water inundates areas up to 146% area more than typical dry seasons and, and the highest average sediment loads (>20,000 mg/L) in September–often more than twice that in the dry season–suggest that this late-season pulse drives most wetland sedimentation, before water levels recede for the incoming dry season. This seasonal-scale sediment capture also depends on ENSO cycles, local precipitation, and modifications to the hydrology by hydropower infrastructure, but despite higher in-channel turbidities during wet La Niña cycles, it is not clear if sediment associated with these cycles reaches off-channel wetlands. Our findings suggest the wetlandscape provides critical sediment retention, an overlooked ecosystem service with implications across the lower river reaches and estuary, but with high degrees of spatial and temporal variability. To reduce excessive sedimentation in this wetlandscape and downstream–including a degraded Ramsar-designated wetland and coral reefs at risk of burial–management and research initiatives should recognize the role of floodplain wetlands in sediment capture and flux buffering.