News Release

Construction Largest Source of Sediment to Mill Creek

Sediment is the most frequently reported impairment of streams and rivers.  Accelerated erosion and transport of sediment is known to reduce soil fertility, increase water-treatment costs, impair aquatic habitat, and decrease storage capacity at downstream impoundments and lakes.  The U.S. Geological Survey, in cooperation with the Johnson County Stormwater Management Program, evaluated suspended-sediment transport and sources in the urbanizing Mill Creek watershed from February 2006 through June 2007.  Streamflow and turbidity were monitored at 9 sites throughout Mill Creek to better understand how different land-use and precipitation affect sediment transport.

The Johnson County Stormwater Management Program is using the data collected from this study to characterize sediment loads from active construction sites and to evaluate the effectiveness of post-construction runoff controls such as stream buffers, detention ponds, and wetlands. The results from this study will be used to target best management practices in areas contributing to increased loading of suspended sediment throughout the county. “The communities in Johnson County are working hard to improve the quality of stormwater runoff throughout the county. This study was conducted to identify the major sources of sediment in a watershed at various stages of urban development. The results of this study have shown that we need to continue to work with the construction industry in an effort to minimize sediment leaving the construction site and ensure compliance with state and local requirements,” said Lee Kellenberger, Johnson County Stormwater Management Program. 

Construction activities were the primary contributor of excess sediment to Mill Creek and its tributaries.  Accelerated sediment transport was observed at sites downstream from urban construction in the Clear Creek basin (located in Western Lenexa and Shawnee) and the south-central Mill Creek basin (Northern Olathe and Central Lenexa).  Substantially less sediment was observed at sites downstream from older established urban areas in Little Mill Creek (Eastern Lenexa and Shawnee) and the headwater portion of Mill Creek (Olathe).  Large impoundments in the watershed (Lake Lenexa and Shawnee Mission Lake) acted to trap much of sediment transported from upstream watersheds.

Sediment transport from small watersheds within Mill Creek were compared to those observed from other large watersheds in Johnson County (Blue River, Cedar, Indian, and Kill Creeks).  In contrast with small sediment loads from older urban watersheds in Mill Creek, Indian Creek(the largest, older urban watershed in Johnson County) exhibited sediment transport similar to areas with substantial urban construction.  Increased sediment transport in Indian Creek may be related to specific construction activities, such as stream-channel disturbance during bridge renovation, or may indicate that elevated sediment transport in large watersheds may lag urban development by years or decades.

The report is now available online at:

http://pubs.usgs.gov/sir/2009/5001/

Further information about sediment sources in Mill Creek is available at:

http://ks.water.usgs.gov/studies/millcreek/