What is Gross Reservoir? A mountain reservoir west of Boulder at 7,300 feet elevation with hiking trails, fishing, and dramatic canyon views. Managed by Denver Water—no swimming, no gas-powered boats. The Gross Dam viewpoint hike is a local favorite. Access via Flagstaff Road or Gross Dam Road.
Overview
Location: Gross Dam Road, west of Boulder
Elevation: 7,300 feet
Size: 440-acre lake
Entry Fee: Free
Hours: Sunrise to sunset
Gross Reservoir sits in a dramatic canyon setting in the foothills west of Boulder. The lake supplies drinking water to Denver, so swimming and gas-powered boats are prohibited. What you get instead: uncrowded hiking trails, good fishing, and mountain scenery without the drive to Rocky Mountain National Park.
Hiking
Gross Dam Viewpoint Trail
Distance: 3 miles round trip | Elevation gain: 600 feet | Difficulty: Moderate
The most popular trail leads to views of the 340-foot dam and the reservoir below. Rocky terrain with some steep sections. The viewpoint at the end is worth the climb—you look straight down at the dam face.
Forsythe Canyon Trail
Distance: 4 miles round trip | Elevation gain: 800 feet | Difficulty: Moderate
Follows Forsythe Canyon Creek through mixed forest. Less crowded than the dam viewpoint trail. Good fall colors. Connects to longer trail options in the surrounding national forest.
South Boulder Creek Trail
Distance: Varies | Difficulty: Easy to moderate
Follows the creek below the dam. Multiple access points allow for different length hikes. More shaded than the dam viewpoint trail.
Fishing
Gross Reservoir is stocked with rainbow and brown trout. Fishing from shore or non-motorized boats (kayaks, canoes, float tubes). Colorado fishing license required. The lake has good depth for trout, especially near the dam and the inlets. Spring and fall produce the best fishing.
Boating
Non-motorized boats only—kayaks, canoes, SUPs, and float tubes. Hand-launch access at the south shore. No gas motors; electric motors allowed with restrictions. The lake is scenic for paddling with canyon walls rising on both sides.
Access & Parking
From Boulder: Take Flagstaff Road west to Gross Dam Road (gravel). About 30 minutes from downtown Boulder.
Parking: Several small lots at trailheads and boat launch. Free but limited—arrive early on weekends. The road is rough gravel; high-clearance vehicles recommended but not required.
Tips
No swimming: It's a drinking water reservoir. Swimming, wading, and body contact with water are prohibited.
Afternoon thunderstorms: At 7,300 feet, lightning is a real concern. Start early and be off exposed trails by early afternoon in summer.
Bring layers: Temperatures can be 10-15 degrees cooler than Boulder.
Frequently Asked Questions
Can you swim at Gross Reservoir?
No. Swimming is prohibited because the reservoir supplies drinking water to Denver.
Is there an entrance fee?
No, access to Gross Reservoir is free.
Can I kayak at Gross Reservoir?
Yes. Non-motorized boats including kayaks, canoes, and paddleboards are allowed.