Lake Hjalmaren is situated in a region with Archaean rocks. The deposits around the lake are dominated by lime-rich clays which makes these areas well suited for cultivation. Large agricultural areas surrounding the lake contribute to the nutrient loading of the water.

Lake Hjalmaren is shallow and of eutrophic character. It consists of four main basins, separated from each other by islands and narrow sounds. The lake is used as a freshwater reservoir, as a recipient of sewage water-mainly from the town Orebro in the westernmost part - and for recreational purposes. The accelerated eutrophication during the last thirty years has especially affected the two basins closest to the town Orebro due to the increased population of the town. The main inflow runs via the town of Orebro to the westernmost basin. Lake Hjalmaren drains via the outlet in the easternmost part to Lake Malaren.

An eighteenth-century observer noted that the water of Lake Hjalmaren was transparent and had a good taste but got thicker during the heavy gales that frequently occurred. The thick water included many nutrients and seemed to be good for the fishes, as they were generally well-fed. Water blooms were common and lasted from mid-June to late autumn (Uggla Hillebrandsson, 1786).