Reservoir · Wyoming · United States · BOR RISE
Grassy Lake
Grassy Lake and Grassy Lake Dam
43.9% full
Capacity
19.08 hm³
Volume stored
8 hm³
Coordinates
44.130°N, 110.819°W
Last updated
2026-09-30
Current fill
43.9%vs 7 days ago
+0.4%gaining
vs 1 year ago
-18.3%year-over-year
7-day trend
→stable
Data source: BOR RISE — US Bureau of Reclamation — daily reservoir storage volume. Capacity is the design (normal full-pool) capacity from the US National Inventory of Dams (NID); fill % = current volume ÷ design capacity. Last reading: 2026-09-30.
Catchment Weather
Past 30 days + 14-day forecast · Open-Meteo
Satellite imagery: Esri, Maxar, Earthstar Geographics. Map: Esri, HERE, Garmin, © OpenStreetMap contributors. 44.130, -110.819 · Open in Google Maps
About Grassy Lake
As of September 30, 2026, Grassy Lake is filled to 43.9% of its total capacity, holding roughly 8 hm³ of water out of a maximum 19 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 (+0.4 pts), and it is 18.3 points below its level on the same date a year earlier.
Across the readings we hold for Grassy Lake — 2188 data points between October 2, 2021 and September 30, 2026 — the level has ranged from a low of 42.9% (September 17, 2026) to a high of 99.8% (June 7, 2022). On average the reservoir is fullest around June and at its lowest around August.
It is operated by BOR primarily for multiple purposes. It lies within the Wyoming basin at 44.130°N, 110.819°W.
Key facts
- Current fill level
- 43.9%
- Water stored
- 8 hm³
- Total capacity
- 19 hm³
- 7-day change
- ▲ 0.4 pts
- Year-on-year
- ▼ 18.3 pts
- Primary use
- Multipurpose
- River basin
- Wyoming
- Operator
- BOR
- Coordinates
- 44.130°N, 110.819°W
- Last updated
- September 30, 2026
Recent readings
| Date | Fill level | Volume (hm³) |
|---|---|---|
| 30 Sep 2026 | 43.9% | 8 |
| 29 Sep 2026 | 43.9% | 8 |
| 28 Sep 2026 | 43.8% | 8 |
| 27 Sep 2026 | 43.7% | 8 |
| 26 Sep 2026 | 43.7% | 8 |
| 25 Sep 2026 | 43.6% | 8 |
| 24 Sep 2026 | 43.6% | 8 |
| 23 Sep 2026 | 43.5% | 8 |
| 22 Sep 2026 | 43.4% | 8 |
| 21 Sep 2026 | 43.3% | 8 |
| 20 Sep 2026 | 43.2% | 8 |
| 19 Sep 2026 | 43.2% | 8 |