The small lake Gröntjärn lacks visible inlets and outlets. It is instead fed by naturally infiltrating groundwater. When the water table rises or falls, so does the lake, by up to 13 metres. Gröntjärn lies in a glaciofluvial delta in a long valley running from west to east. When it rains, the aquifer in the delta is quickly filled by water draining from higher elevations. The water level can drop just as quickly when groundwater drains away to the east. Steeply sloping sand beaches show clear waterlines at the lake. 

""Photo: Margareta Eiserman.