The U.S. Geological Survey operates the one gaging station (USGS Gage 06188000) on the Lamar River in cooperation with Yellowstone National Park. The gage is located just above the confluence with the Yellowstone River and has daily flow data dating back to 1923. A portion of the discharge record spanning from Fall 1969 to Summer 1988 is missing.
The Lamar River shows a characteristic snow-driven hydrograph, where peak flows can be two orders of magnitude (100 times) higher than baseflows. The average of annual peak flows between 1924 and 2021 is 8,085 cfs occurring on average on 4 June (day 155 of the year). Minimum annual daily flows over this same period of record averaged 70 cfs. Water flows in the Lamar River in 2021 were under the 25th percentile long-term (1924–2020) flow for the site following peak flow. This occurred as a result of a warmer, drier summer in this region of the park. In 2021, peak flow was 5,960 cfs and occurred on 5 June (day 156 of the year). June received less than 20% of the precipitation compared to the 30-year average (1991–2020) while experiencing maximum temperatures at Tower Falls recorded at 6°C over the 30-year average (1991–2020).