Is the Pre-Lake Bottom ‘Landscape’ Causing Persistent Melt Features in the Ice of Lake Helena

Abstract

Lake Helena is a popular winter recreation area utilized by ice anglers, skaters, and cross-country skiers. It is also known for potential dangerous areas of open water, pressure ridges, and ‘melt holes’. The lake was formed when Hauser Dam was built across the Missouri River in 1911. Historic USGS topographic maps from 1899, show that prior to the dam, this lowest area of the Helena Valley was occupied by a stream channel, extensive wetlands, ponds, and alluvial fans gently sloping from the valley edge. The aim of my study was to determine if ice-melt features (holes, leads, gas bubbles) appear persistently in the same locations throughout the winter season and from year to year. If so, I wanted to determine if these melt features aligned spatially with specific pre-impoundment landforms such as springs, wetlands and channels. To do this, I obtained repeat high resolution (10m), multispectral imagery from the Copernicus Sentinel-2 L2A satellite.The data set spans nine winter seasons (Oct-April) from 2018-2026. I selected all images of Lake Helena that were both cloud free and had a continuous, shore-fast covering of ice. I mapped any visible holes and open areas from each image into ArcGIS. I then performed proximity analyses to determine if they formed repeatedly in the same place and tested the hypothesis that melt holes in the ice align spatially with known underlying structures. My results include statistical output and a map of persistent melt areas that may help winter recreationalists avoid dangerous areas of the lake.