Mill Creek Sediment

How does urbanization affect sediment transport in streams and lakes, and why do changes in sediment transport matter?

During urban construction, removal of surface vegetation and excavation of soils for building and road foundations greatly increase soil erosion during rainfall events. These soils are transported from construction sites and can be redeposited on land surfaces, floodplains, and streambeds and in lakes. When the construction phase is finished, impervious surfaces (streets, parking lots, building roofs) route rainwater directly to streams, resulting in larger, faster streamflows that can increase the transport of deposited sediments and incise or widen stream channels. Changes to natural streams as a result of urbanization can result in property loss, reduction in biological diversity, siltation of downstream reservoirs, and increased water treatment costs.

How is sediment transport being studied in Johnson County?

The vast majority of sediment moves through streams during sporadic, high flow events which occur during and after periods of heavy rainfall. This is especially true in small, urban streams in which runoff conditions can persist for only hours. Historically, sediment-related impairments to streams and lakes have been assessed by comparing the results of periodically collected sediment samples. However increased recognition of the time-scales in which sediment is transported, as well as the deployment of sensors that collect water-quality data continuously within streams have allowed sediment impairments to be assessed much more accurately. Since 2002, the USGS has installed and operated continuous stream-level and turbidity sensors to quantify if, and to what extent climate and land-use change have changed sediment transport in streams.