During 2018, Kilauea Volcano, on the Island of Hawaiii, had a large effusive eruption (~1 cubic kilometer of lava) on the lower East Rift Zone that caused widespread destruction (Neal and others, 2019; Dietterich and others, 2021). This lower flank eruption was accompanied by one of the largest collapses of the summit caldera in two hundred years, with portions of the caldera floor subsiding more than 500 m (Anderson and others, 2019; Neal and others, 2019). On July 25, 2019, approximately one year after the summit collapse sequence, a small pond of water was first observed in the deepest portion of the collapse pit, within Halema?uma?u crater (Nadeau and others, 2020). The water level rose gradually over the next 17 months, and was measured on a routine basis with a laser rangefinder by the U.S. Geological Survey's Hawaiian Volcano Observatory. The water level rose at rates of 5-15 cm day-1, achieving a maximum lake depth of approximately 50 m by December 2020. The volume of water in the conical collapse pit was approximately 900,000 m3 by that time. The lake water originated from groundwater seepage into the new collapse crater, and represented the water table slowly recovering following the 2018 collapses (Nadeau and others, 2020). This water lake period ended abruptly on December 20, 2020, as a fissure eruption poured lava into the bottom of Halema'uma'u, boiling off the lake over a span of 1.5 hours. Water level is a fundamental measure of crater lake activity, and the 2019-2020 water lake was the first known instance of a significant body of surface water at the summit of Kilauea in over 200 years (Nadeau and others, 2020). The water-level data presented here provide unique constraints on the properties of the volcano's summit hydrology (Ingebritsen and others, 2020). The water lake also had potential implications for hazards at Kilauea, which has experienced explosive eruptions thought to involve the interaction of rising magma with ground or surface water (Swanson and Houghton, 2018). Fluctuations in water level have been a precursor to hazardous changes at other volcanoes with crater lakes (Hamling, 2017). Close monitoring of the water level at K?lauea was performed to help identify changes that might precede hazards at the summit. This data release provides water level measurements and estimated lake surface area and volume during the lake's growth, between August 2019 and December 2020. References Anderson KR, Johanson IA, Patrick MR, Gu M, Segall P, Poland MP, Montgomery-Brown EK, Miklius A. 2019. Magma reservoir failure and the onset of caldera collapse at K?lauea Volcano in 2018. Science, doi:10.1126/science.aaz1822 Dietterich H, Diefenbach A, Soule AS, Zoeller M, Patrick M, Major JJ, Lundgren P. 2021. Lava effusion rate evolution and erupted volume during the 2018 Kilauea lower East Rift Zone eruption. Bulletin of Volcanology 83:1-18. Hamling IJ. 2017. Crater lake controls on volcano stability: Insights from White Island, New Zealand. Geophysical Research Letters 44, 11,311-11,319. Ingebritsen SE, Flinders AF, Kauahikaua JP, Hsieh PA. 2020. Modeling groundwater inflow to the new crater lake at K?lauea Volcano, Hawai?i. Groundwater 59, https://doi.org/10.1111/gwat.13023 Nadeau PA, Diefenbach AK, Hurwitz S, Swanson DA. 2020. From lava to water: A new era at K?lauea. Eos 101, https://doi.org/10.1029/2020EO149557. Neal CA, Brantley SR, Antolik L, Babb J, Burgess M, Calles K, Cappos M, Chang, J.C., Conway, S., Desmither, L., Dotray, P., Elias, T., Fukunaga, P., Fuke, S., Johanson, I.A., Kamibayashi, K., Kauahikaua, J., Lee, R.L., Pekalib, S., Miklius, A., Million, W., Moniz, C.J., Nadeau, P.A., Okubo, P., Parcheta, C., Patrick, M.R., Shiro, B,, Swanson, D.A., Tollett, W., Trusdell, F., Younger, E.F., Zoeller, M.H., Montgomery-Brown, E.K., Anderson, K.R., Poland, M.P., Ball, J., Bard, J., Coombs, M., Dietterich, H.R., Kern, C., Thelen, W.A., Cervelli, P.F., Orr, T., Houghton, B.F., Gansecki, C., Hazlett, R., Lundgren, P., Diefenbach, A.K., Lerner, A.H., Waite, G., Kelly, P., Clor, L., Werner, C., Mulliken, K., Fisher, G. 2018. The 2018 rift eruption and summit collapse of Kilauea Volcano. Science 363, eaav7046. Swanson DA, Houghton B. 2018. Products, processes, and implications of Keanak?ko'i volcanism, K?lauea Volcano, Hawai'i, in Poland, M.P., Garcia, M.O., Camp, V.E., and Grunder, A., eds., Field Volcanology-A Tribute to the Distinguished Career of Don Swanson: Geological Society of America Special Paper 538, p. 159-190, https://doi.org/10.1130/2018.2538(07).