The St. Vrain River is a tributary of the South Platte River located in northern Colorado. It flows through the Rocky Mountain foothills and is approximately 70 miles long. The river begins in the Indian Peaks Wilderness and travels through several towns, including Longmont and Lyons, before joining the South Platte. The St. Vrain River is known for its scenic beauty and recreational opportunities. It offers activities such as fishing, kayaking, and hiking along its banks. The river also plays a vital role in the local ecosystem, supporting various wildlife and plant species in the region.