Tumblesom is a private fish and wildlife pond located in Lyons, Colorado, along the South St. Vrain Creek. Constructed in 1918, this rockfill dam stands at 19 feet tall and spans 186 feet in length, with a storage capacity of 149 acre-feet. The dam serves multiple purposes, including providing habitat for fish and wildlife, as well as offering recreational opportunities for visitors.

Managed by the Colorado Department of Water Resources, Tumblesom has a low hazard potential and is in satisfactory condition as of the last inspection in October 2020. With a moderate risk assessment rating, the dam is equipped with an uncontrolled spillway and has a maximum discharge capacity of 96 cubic feet per second. While no emergency action plan is currently in place, the dam is regularly inspected every six years to ensure its safety and functionality.

Located in the Omaha District, Tumblesom is overseen by private owners and falls under state jurisdiction for regulation, permitting, inspection, and enforcement. The dam plays a crucial role in supporting the local ecosystem and providing a scenic backdrop for recreational activities, making it a notable water resource and climate enthusiast destination in Boulder County, Colorado.