A small islet lies shortly upstream of the dam, where the James River previously split into two channels.

The valley also contains many traces of alluvium, mainly deposited during the last ice age when the area was heavily glaciated.

The site selected offered the required storage with the smallest dam and with close access to embankment materials; however, the underlying rock and soil was both unstable and pervious.

If this had not compensated for the pervious material, the required excavations to stabilize the dam would have been over 120 feet (40 m) to solid bedrock.

The outlet works, which consist of four high-pressure floodgates, were provided by the Hardie-Tynes Manufacturing Company of Birmingham, Alabama, at a cost of $72,995.

The dam embankment contains 8,667,000 cubic feet (245,400 m3) of material, primarily dirt, rock, gravel and riprap.

The Jamestown Reservoir typically holds 30,000 acre-feet (37,000,000 m3) of water, with a normal surface area of 2,095 acres (3.273 sq mi).

[1] The Jamestown Dam's primary purpose is flood control, and has prevented approximately $36 million in flood-related damages.

[1] Besides flood control, the 8-mile (13 km) long, 2,095-acre (3.273 sq mi) Jamestown Reservoir was also created for the secondary purpose of recreation, which mainly involves camping, boating, fishing, and birding managed by the Stutsman County Park Board.

The lake has approximately 45 miles (72 km) of shoreline and is referred to as "one of the greatest migratory waterfowl flyways in North Dakota."

Common fish species in the reservoir include pike, walleye, crappie, bluegill, smallmouth bass, muskie and bullhead.

The bedrock is Pierre Shale which is normally hard, dark gray, thinly bedded and highly jointed claystone or siltstone.