2027 15th Annual Rock CF Rivers Half Marathon & 5K

Grosse Ile is an island in the Detroit River, and each late March the Rock CF Rivers Half Marathon takes over its flat, quiet roads — 2026 marks the 15th annual running. It's a spring race with a real edge: a field of about 1,300 finishers, mornings that the five-year averages put somewhere between the mid-30s and upper-40s, and a cause that keeps pulling people back.

The half runs a USATF-certified, pancake-flat loop on pavement, tracing the island with open views across the Detroit River. There's essentially no climbing, so it's a course built for rhythm and quick times — the winners come through around 63 minutes. Start and finish share the same spot at Grosse Ile Middle School, which keeps race morning simple.

Race day runs on community: 100% of proceeds support the Rock CF Foundation's work for people living with cystic fibrosis, and the crowd reflects it — roughly four in ten runners come back in a later year, and entrants travel in from about 17 states. A 15K and a 5K round out the morning, plus a virtual option, and a long-sleeve tech shirt comes with your entry.