Sediment transport to streams and lakes in
Johnson County, Kansas
The U.S. Geological Survey, in cooperation with the Johnson County Stormwater Management Program has studied sediment
transport in Johnson County streams and lakes to better
(1) understand how changes from agricultural to urban land use alter sediment in streams and lakes,
(2) characterize how these uses may affect lake storage and biological integrity of streams, and (
3) evaluate the effectiveness of management practices designed to reduce sediment transport to streams and lakes,
so that the county can best manage its water resources.
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.
USGS sediment work has (1) assessed the effects of urbanization on sediment transport in small streams,
(2) examined how results from small basins apply to larger streams in Johnson County,
(3) characterized the effect of selected management practices on sediment transport to county streams and lakes,
and (4) quantified sediment accumulation and consequent reductions in water storage in selected Johnson County lakes.
Construction-related affects on sediment transport
Streams directly downstream from ongoing and recent construction activity were studied to assess the effect of
urban construction on sediment transport. Results indicate that even under extensively applied erosion and
sediment controls, streams downstream from construction activities transported up to 55 times the amount of sediment
(per unit area) than streams downstream from similarly-sized urban and rural basins in the county.
Streams downstream from construction sites had larger turbidity values and sediment concentrations for longer
periods of time, and increased deposition of fine sediments on streambeds; factors which have been found to impair
the integrity of aquatic ecosystems.
Sediment transport from small to large streams
The effects of landscape alteration on water quality are most directly assessed in small streams, in which changes
in water quality may be apparent during and immediately after upstream changes in land use. However linkages between
stream-water quality and human activities become more complex in larger basins as land use becomes more diverse,
and as biological, chemical, and physical in-stream processes transform inputs from landscapes. Comparison of sediment
transport among small and large streams in Johnson County indicated that while sediment loads dramatically increased
as a direct result of construction in small streams, larger streams appear to respond more slowly to urbanization.
Indian Creek, the only large (63 mi2), nearly completely urban stream within Johnson County, transported nearly
double the amount of sediment compared to similarly sized, urbanizing and rural basins, and had from 2 to 7 times
the sediment yields of smaller, predominantly urban stream-sites within the county. Consistently large sediment
yields observed at Indian Creek since 2004 indicate that increased sediment transport is likely caused by a large-scale
basin and stream-channel response to urbanization (rather than specific construction activities). Increased sediment
transport in Indian Creek is likely caused by more frequent stormflows that erode streambed and streambank sediments.
Effects of management practice
Selected practices which help reduce sediment transported to streams and lakes in Johnson County were evaluated by USGS
sediment studies. These practices include erosion and sediment controls at and downstream from construction sites, as
well as impoundments constructed on streams. Although much work as been done on plot-scale effects of BMPs, little is
known about the timing, and degree to to which managment effects sediment transport at the watershed scale. Management
practices were evaluated by comparing data among watersheds, as well as a portion of the study in which turbidity sensors
were installed up- and downstream from an artificial wetland and sediment forebay upstream from Shawnee Mission Lake (see map).
Study results indicated that despite existing sediment and erosion controls at construction sites, ongoing construction
was the largest source of sediment in small streams in Johnson County. Comparison of results through time and among
sampling sites showed that precipitation and site-specific factors, such as the position of the construction site relative
to a stream, may affect the amount of sediment transported to county streams and lakes. Comparison of data up- and downstream
of the sediment forebay and artificial wetland on a tributary entering Shawnee Mission Lake indicated that the sediment
forebay removed approximately 33 percent of the incoming sediment load into Shawnee Mission Lake, with no measurable removal
from the downstream wetland. Management practices are often designed to encourage sediment deposition by slowing water
velocities, however silt and clay soils which predominate in Johnson County can remain in suspension for extended periods of time,
making managment of eroded sediments difficult, and increasing the importance of measures that can minimize soil erosion
Studies found that relatively large surface water impoundments (such as Lake Lenexa and Shawnee Mission Lake) accumulate
incoming sediments, substantially decreasing the amount of sediment transported to downstream surface waters.
Sediment transport to lakes
Reservoirs slow the velocity of incoming streams, allowing the sediments carried by those streams to accumulate
on the bottom of the impoundment. Sediment accumulation decreases the water storage capacity of reservoirs,
eventually making the lake unsuitable for recreation, drinking water, and other designated uses.
In separate studies, cores of the bottom sediments have been collected within Cedar Lake, Lake Olathe, Gardner City Lake,
Edgerton City Lake, and Shawnee Mission Lake within Johnson County Kansas (Mau, 2002; Juracek, 2004,
Lee and others, 2009, USGS Reservoir Sediment Studies). Three lakes (Cedar Lake, Gardner City Lake and Lake Olathe)
had surveys completed during reservoir construction, allowing the estimation of sediment accumulation since dam construction.
As of 2000, sediment accumulation had reduced the capacity of Cedar Lake by approximately 50 percent,
Gardner City Lake by 12 percent, and Lake Olathe by 10 percent. Cedar and Gardner City Lake watersheds each had similar
sediment yields (0.89 and 0.85 acre-ft/year/mi2 respectively), Lake Olathe had a substantially smaller yield
(0.42 acre-ft/year/mi2) in part because of deposition in Cedar Lake upstream from Lake Olathe. In Shawnee Mission Lake, coring results indicated that as of 2006, sediment accumulation had not noticeably increased near the dam as a result of upstream residential construction activities.
How does this benefit Johnson County?
Studies are used by city and county officials to better understand where, and at what scale impacts from landscape
disturbance can be controlled. Data help city and county officials understand (1) the degree to which water quality
in streams have been altered, (2) how conditions compare to state and federal criteria, and (3) how stream-water quality
conditions compare to upstream land uses and practices. This understanding is used to make more informed decisions that
will help protect stream-water quality and stream ecosystems.
What are the unresolved questions?
Results in the highly urbanized Indian Creek basin indicate that stream-channel erosion is likely a substantial
source of sediments, but the future locations and degree to which the stream channel is changing, and the potential affects
this movement may have on infrastructure and property surrounding Indian Creek are unknown.
Another unresolved question is the extent to which sediment-related impairments impact the ecology of Johnson County streams.
While marked decreases in the health of aquatic ecosystems have been observed in urban streams,
the reasons for this decline are less well understood. Studies that evaluate the ecological quality of
streams at specific thresholds of urbanization and in relation to indicators of sediment impairment could help
determine whether management practices which reduce sediment impairments will improve the health of stream ecosystems.