Swimming at Hop Brook Lake happens only inside a short posted season and only when the Corps of Engineers says the beach is open, and that is not a formality here. The swimming area closed in May 2024, reopened, then closed again on July 1, 2024 after heavy rain pushed bacteria counts past the limit. Follow-up testing found extremely high cyanobacteria and the beach stayed shut for the rest of the 2024 season. It had also been closed in July 2023. Read the signs at the gate before you plan a swim, because no third party listing anywhere reflects this week's water testing.

The lake is the recreation pool behind Hop Brook Dam, a U.S. Army Corps of Engineers flood control project on Hop Brook in the Naugatuck River basin. The dam went up between December 1965 and December 1968 at a cost of about $6.2 million, an earthfill structure 520 feet long and 97 feet high, one piece of the system Congress authorized after the August 1955 flood tore through the Naugatuck valley. The impoundment covers roughly 21 acres and runs about 18 feet at its deepest. The Corps holds 536 acres of public land around it, spread across Middlebury, Naugatuck and Waterbury.

Entry is free. The Corps states that entrance into the park and activities in the park carry no charge, and Recreation.gov notes that user fees are no longer collected at the gate. The only money that changes hands is for reservations, all of them handled by the Corps through Recreation.gov or at 877-444-6777: the two large picnic shelters at $135 per day, the two small ones at $100 per day, and the 2.99 acre ball field at $75 per day. Nobody charges for parking, and nobody charges for the beach.

The hours trip people up. The recreation area opens the weekend before Memorial Day weekend and closes the weekend after Labor Day, running 8:00 a.m. to 6:00 p.m., and the gate is open to vehicles on Fridays, Saturdays, Sundays and holidays only. Monday through Thursday cars cannot get in. A more recent Corps notice described the grounds as open daily, so call the park office at 203-729-8840 if your trip falls midweek.

Past the beach there are more than seven miles of trails through the Corps property, well used by hikers and by mountain bikers. The trail network is established enough to host cross country mountain bike racing, and the staging areas and singletrack see steady traffic through the warm months. Four picnic shelters, a ball field, volleyball, horseshoes and restrooms round out the day use area. This is day use only with no overnight camping. Motorboats are not permitted and electric trolling motors are, so canoes, kayaks and rowboats have the 21 acre pool to themselves. Connecticut DEEP stocks the impoundment with brown, brook and rainbow trout, twice in the spring of 2026 alone. Dogs are banned from the beach and leashed everywhere else. Drones are prohibited. The park office sits at 4 Straits Turnpike on Route 63.