Big Johnstone Lake is a remote glacial lake located approximately 42 km (26 mi) southeast of Seward, Alaska. Situated at an elevation of 226 ft (69 m), the lake lies south of the Sargent Icefield and Excelsior Glacier. As of 2018, Big Johnstone Lake had expanded to a length of 6.3 miles (10.2 km) due to glacial retreat.

The lake is fed by the Excelsior Glacier, which has been rapidly receding in recent decades. Large icebergs calving from the glacier can be observed floating in the lake, creating a dynamic and ever-changing paddling environment. The western tributary of the glacier, still called Excelsior Glacier, lost about 30% of its length between 1994 and 2018.

Access to Big Johnstone Lake is extremely challenging, as it is located in a remote wilderness area with no road access. In the 1980s, some adventurous individuals attempted to reach the area by small boat or aircraft to stake land claims, but several fatalities occurred due to the hazardous conditions.

The lake drains via the Big Johnstone River, which flows about 1.3 miles (2.1 km) to Johnstone Bay on Blying Sound in the Gulf of Alaska. The surrounding landscape features steep, glaciated mountains and dense coastal forests typical of the Kenai Peninsula.

Paddlers venturing onto Big Johnstone Lake should be aware of the potential for calving icebergs, rapidly changing weather conditions, and very cold water temperatures year-round due to the glacial source. The lake's remote location means rescue services are extremely limited, so self-sufficiency and wilderness experience are essential.

Wildlife in the area may include bears, whales (in nearby coastal waters), and various bird species. The lake offers opportunities to observe the dramatic effects of climate change on Alaska's glacial landscape firsthand.