Nelson Reservoir

As of October 1, 2026, Nelson Reservoir is filled to 73.6% of its total capacity, holding roughly 72 hm³ of water out of a maximum 98 hm³. At this level the reservoir is within its normal operating range.

Over the most recent week on record the fill level has held broadly steady (-1.1 pts), and it is 3.8 points below its level on the same date a year earlier.

Across the readings we hold for Nelson Reservoir — 1819 data points between October 2, 2021 and October 1, 2026 — the level has ranged from a low of 43.7% (August 22, 2022) to a high of 99.3% (May 3, 2024). On average the reservoir is fullest around May and at its lowest around February.

It is operated by USACE primarily for multiple purposes. It lies within the Montana basin at 48.530°N, 107.519°W.