Fall Creek Watershed and Geist Reservoir

• Watershed and Setting

• Reservoir History and Role in Drinking Water Supply

• Geist Reservoir – At a Glance

Watershed and Setting

             Fall Creek watershed has a drainage area of 318 mi², 215 mi² of which is above Geist Reservoir (Martin, 1995).  Fall Creek flows from its headwaters in Henry and Delaware Counties down through Madison, Hamilton, and Hancock Counties and into Geist Reservoir. The southern portion of the reservoir and dam is located in the northeastern corner of Marion County (Fig. IV-10). The tributaries of Fall Creek above the Reservoir are Honey Creek, Sugar Creek/Deer Creek, Sly Fork, Sugar Fork, Mud Creek/Little Creek, Lick Creek, Spring Branch, Prairie Creek, Foster Branch, Flat Fork Creek, and Manifold/McFadden Ditches. Thorpe Creek, Bee Camp, Dry Branch and Fall Creek flow directly into the Reservoir. Fall Creek flows from the reservoir at the southwestern end and discharges into the White River approximately 17.6 miles downstream at the IUPUI campus. Martin (1995) interpreted maps from Meyer (1979) and Smith (1983) to conclude Fall Creek is a gaining stream, in other words groundwater feeds the stream.  The watershed upstream of the reservoir can be divided into 15 smaller sub-watersheds (Fig. IV-10; Table IV-2).  The major tributary contributing to Fall Creek is Lick Creek, which drains a watershed area of 38.2 mi² (Martin, 1995). Both rural and urban hydrology influences the watershed.  According to the land use model discussed in the Land Use Characterization section of this report (Section VIII), nearly 60% of the land use in the watershed is agriculture (Fig. IV-11).  Developed areas in the watershed is highest in the north-central part of the watershed, where the city of Anderson is located, and in the lower and middle portions of Geist Reservoir where continued development is occurring.    

            The Fall Creek watershed is located in the Tipton Till Plain and has a mostly flat topography.  The upland soil associations in the watershed are predominantly Crosby-Treaty-Miami Association and Crosby-Cyclone-Miamian Association. Crosby upland soils are somewhat poorly drained, while Miami soils are well drained. Fox-Ockley-Westland and Miami-Crosby-Treaty Associations occur in valleys and are well-drained, moderately deep soils. Fox soil associations overlie sand and gravelly sand (Fig. IV-12). Miami and Crosby soils are developed in loess and the underlying calcareous glacial till, while Fox soils form in loamy outwash and the underlying gravely sand and sand.   

            Unconsolidated deposits in the Fall Creek Watershed are complex (Fig. IV-13). The primary deposit is loam till. Stream valleys consist of lake sands and alluvium. Glacial ice-contact sediments bisect the watershed in a north-south direction (Fig. IV-13) affecting the flow of some streams. Unconsolidated deposits in the watershed are up to 350 feet thick. Bedrock is overlain by unconsolidated deposits in most areas, but is exposed at the surface near Pendelton. Bedrock is comprised of Paleozoic limestones, dolomites and shale (Fig. IV-14).           

            Mean annual precipitation for Geist Reservoir and its surrounding area for 1971-2000 was 42.85 inches.  The months with the lowest precipitation were January and February, both with 2.42 inches.  May had the highest average monthly precipitation of 4.86 inches.  Monthly mean temperature normals for the period 1971-2000 ranged from 25.3°F in January to 74.2°F in July (PAMG, 2003).  Monthly streamflow was averaged for the 1941-2002 period at the USGS gauging station at Fortville, IN (03351500) within the Fall Creek watershed (Fig. IV-15).  The month with the highest streamflow is March with a mean discharge of 296 ft³/s.  September has the lowest mean streamflow with an average discharge of 56.8 ft³/s (USGS, 2003). 

Figure IV-15

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Reservoir History and Role in Drinking Water Supply

The Indianapolis Water Company (IWC) first conceived the development of the Geist Reservoir (GR) in 1923, however actual construction did not begin until 1941. Construction was completed on the GR in 1944. The primary purpose for its development was to generate a more consistent source of water supply to the IWC’s Fall Creek Water Treatment Facility, which is located approximately eight miles southwest of the GR adjacent to Fall Creek at Keystone Avenue. The GR was largely an isolated body of water for over three decades. In approximately 1981/1982, IWC started a program of real estate development surrounding the reservoir. Today, GR (Fig. IV-16) has significant home developments, which nearly cover its 35 miles of shoreline. The GR provides a recreational resource to this community for swimming, boating, and fishing. The City of Indianapolis purchased GR in 2002 and now owns this water resource. The utilization of the GR as a water source for the Fall Creek Treatment Facility involves a number of parameters. Fall Creek essentially feeds the GR and then continues below the dam to a confluence with the White River. During periods of low precipitation, a valve at the GR dam is controlled/opened to maintain a flow to meet the demand at the Fall Creek treatment plant, and to maintain a minimum flow over the low head dam at Keystone Avenue. The flow over the Keystone Dam is monitored by way of a USGS gauge, which is installed above the dam. As the Keystone dam is adjacent to and below the millrace serving the Fall Creek water plant intake, the referenced USGS gauge is critical to the GR dam valve control. Complicating this control is an approximate 20-hour cycle from an event at the GR dam to the Keystone dam. Due to the eutrophic nature of GR, there have been historic taste and odor compounds exhibited in the reservoir, which are significant. While the Fall Creek treatment plant is not technologically equipped to address the levels of taste and odor compounds due to algae blooms in the GR, there are mitigating elements available to the treatment operation. These include use of groundwater for blending and powdered activated carbon.  

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Geist Reservoir – At a Glance

·         Ownership – The City of Indianapolis

·         Original purpose – Water supply

·         Date into service – 1943

·         Water surface area – 1900 acres; 1478 acres¹

·         Maximum depth – 48 feet

·         Watershed area above dam – 215 square miles

·         Storage capacity – 6.9 billion gallons

·         Dependable water supply yield – 21.8 MGD

·         Rated capacity of Fall Creek plant – 32 MGD[1]

·         Permanent pool elevation – 785.0 feet M.S.L.

·         Overall dam length – 1900 feet

·         Dam height above valley – 40 feet

·         Water depth at dam – 26 feet

·         Type of embankment structure – Earthen fill

·         Type of outlet structure – Open Crest Spillway
 

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