Where Does All the Water Go on Simonton Lake?

Simonton Lake in Elkhart County is not an isolated, closed body of water. Like most natural lakes, it maintains a delicate balance of water input and output, keeping lake levels steady (or fluctuating within limits). Understanding how water enters and leaves the lake is key to managing water quality, shoreline erosion, and flood control.

Inflows: How Water Enters the Lake
Groundwater and Springs
Simonton Lake is primarily spring-fed, meaning much of its water comes from groundwater seeping into the lake substrate. This source is relatively stable over time and less subject to short-term weather fluctuations. According to the Simonton Lake HOA’s environmental resources, no major rivers or streams flow into the lake; instead, underground springs supply a baseline inflow. 

Surface Runoff
Rainfall and snowmelt create surface runoff across the surrounding watershed. That water flows over lawns, streets, roofs, and fields, picking up sediments, nutrients, and pollutants before entering the lake. In heavy rain events, runoff can be a significant contributor to lake water volume, especially if impervious surfaces are extensive.

Minor Tributaries and Storm Drains
Although there are no large inflowing rivers, small creeks or drainage ditches may carry stormwater into the lake, especially after heavy precipitation. These minor tributaries are often intermittent and may dry up during dry periods.

Outflows: How Water Leaves the Lake
Lilly Creek / Osolo Ditch
The primary outflow from Simonton Lake is via Lilly Creek (also called Lilly Ditch or Osolo Ditch), which carries excess water southward from the lake. From there, it eventually empties into the St. Joseph River. The lake’s outflow through Lilly Creek helps manage water levels during wet periods. 

Weir Control
To help regulate outflow and protect lake levels, a weir structure has been proposed (and in planning) for Lilly Creek. A weir is a barrier across a channel designed to allow controlled overflow at a designated elevation. In times of high lake levels, excess water spills over the weir; in low water periods, the weir helps retain water in the lake until it rises to the threshold. This approach helps stabilize fluctuations in lake levels. 

Water Balance and Fluctuation
The health and level of Simonton Lake depend on the delicate balance between input and output:

• During dry spells, groundwater and limited runoff may not fully offset losses through the outflow or evaporation, causing lake levels to drop.
• During heavy rain or snowmelt, surface runoff may overwhelm the capacity of Lilly Creek or the weir, causing temporary flooding or shoreline inundation.
• The weir structure plays a crucial role in ensuring the lake does not drop too low or overflow excessively, especially in extreme years.

Seasonal influences like precipitation, snowmelt, and evapotranspiration (water lost through plant and soil surfaces) further affect how much water remains in the lake versus how much flows out.

Why It Matters to Lake ResidentsUnderstanding where the water goes is more than academic—it’s essential for:

• Lake level management: Residents want stable water levels to protect docks, lakeshore properties, and boat access.
• Erosion control: Sudden or sustained high water levels can erode shorelines; low levels can expose sediments and change nearshore habitat.
• Water quality: Runoff carries nutrients and pollutants; how quickly water is flushed out affects the retention and dilution of contaminants.
• Flood prevention: Properly designed outflow structures (like the weir) help buffer against flood damage in extreme weather.

Simonton Lake's water moves in through groundwater springs and runoff, and it flows out through Lilly Creek. A controlling weir helps stabilize levels by acting as a gatekeeper, allowing excess water to exit only when levels rise above a set threshold. This hydrologic cycle shapes both the lake’s ecology and its usability by residents.