English: Oval Spring is a geyser in the Sawmill Group of Yellowstone's Upper Geyser Basin. It is located about 40 meters north-northeast of Sawmill Geyser and 13 meters north-northwest of Penta Geyser. The circular basin is about 9 meters in diameter. A centrally-located, irregularly elongated vent is oriented northeast-southwest and is about 1 by 2 meters in size. Much of the basin is floored by geyserite, which varies in surface morphology from nodulose to smooth-but-slightly-undulating to terracetted. Shallow, irregularly dendritic channels are present in the geyserite floor that drain water toward the central vent.

Water levels in Oval Spring vary depending on the nature of eruptive activity in other Sawmill Group geysers. Observed conditions include: low to very low water levels (but still visible), moderately high water levels, and a water-filled basin with slight overflow. Observed behaviors range from calm pools to pulsing/undulating to splashing eruptions (see pictures elsewhere in this photo album).

In early 2017, nearby Sawmill Geyser, which normally erupts frequently, ceased activity - it has been dormant ever since. Changes also occurred in all other Sawmill Group features. The dominant behavior in the group involves frequent eruptive play from Sawmill Geyser, Tardy Geyser, and Spasmodic Geyser. During some past seasons, Penta Geyser and Churn Geyser have dominated the group's activity. Since the Sawmill dormancy started, Spasmodic has been the most active member of the group - it continues to have frequent eruptions. Oval Spring has also been affected.

From 2013 to 2016, eruptions at Oval Spring were often reported. The last reported eruption was in early fall 2016. The above photo shows Oval Spring's condition in summer 2017 - no eruptions, calm water having a greenish-blue color and sitting at a moderate level, plus some yellowish bacterial mats growing on parts of the geyserite basin floor. The color changes indicate the feature has cooled (see thermal photos from 2016 in this album when Oval Spring was still hot).

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See video of Oval Spring:

www.youtube.com/watch?v=8qjkDaRJxIg