Sources of suspended sediment in the
Mill Creek watershed, Johnson County, KS

Increased suspended sediment transport associated with storm runoff at Mill Creek at 87^th Street Lane (MI4)

Introduction

The Mill Creek watershed drains 62.8 square miles within the cities of Olathe, Lenexa, and Shawnee (Figures 1, 2).  The watershed has undergone residential and commercial development primarily in the east (Lenexa and Shawnee) and south (Olathe) and is among the most rapidly developing areas in Johnson County, KS. New construction is occurring in many of the undeveloped portions of the basin.

Figure 1. Streams, municipalities and site locations in the Mill Creek watershed.

Figure 2. Land use in the Mill Creek watershed.

Existing and ongoing urban development can cause substantial changes to the streamflow and sediment transport characteristics of area streams.  Increased impervious surface cover (rooftops, sidewalks, roads, parking lots) associated with urban development decrease the connection between streams and groundwater, directing precipitation directly into receiving streams.  These changes work to increase the flood risk and erode stream channels.  New construction activities remove vegetation from surface soils, destabilizing the soil structure, and allowing for increased erosion during precipitation events. 

Purposes

The USGS, in cooperation with the Johnson County Stormwater Management Program, is conducting a study to characterize the magnitude and sources of suspended sediments in the Mill Creek watershed.

Specifically, the study will

-         Estimate concentrations and loads of suspended sediment at 8 sites within the Mill Creek watershed.

-         Estimate sources of suspended sediment using chemical tracer techniques and stormflow sediment dynamics. 

Approach

Water-quality and stage sensors are operated at 9 sites (Figure 1)  in the Mill Creek watershed. Water quality sensors continuously (every 5 minutes) measure:

1.   Gage height (depth below stream surface)

2.   Specific conductance (electrical charge carried by streamwater)

3.   Water temperature, and

4.    Turbidity (murkiness of the water)

Data are used to estimate streamflow and suspended-sediment transport during stormflow conditions.

Instantaneous measurements of streamflow and suspended sediment concentration are statistically related to continuous gage height and turbidity measurements in order to model streamflow and suspended sediment concentration every 5 minutes at all 9 sites ( Figure 3 ).

Benefit

This study will characterize subwatersheds/land uses which are the dominant sources of suspended sediments in the Mill Creek basin.  The information will be used by the Johnson County Stormwater Program to aid in the implementation of BMPs to decrease sources of sediment to the stream.