The Grote Laak is a non-navigable waterway which rises in Leopoldsburg. It has a total trajectory of 29.7 km, flowing through the municipalities of Ham, Tessenderlo and Laakdal to join the Grote Nete. This confluence area coincides with the nature reserves Zammelsbroek and Trichtelbroek, part of the valley of the Grote Nete. Parts of this valley fall into the Natura 2000 Habitats Directive area, ‘Upper reaches of the Grote Nete with Zammels Broek, Langdonken and Goor’, in which conservation and restoration of the valley are the principal objectives. The Flemish flood risk strategy is also applicable here, to increase the natural dynamics of the Grote Nete. Although there are still many meanders that play an important role as environmental elements, a large number of them have been straightened out in the past, relics of which are still visible in the landscape.
The undulating terrain alternates agricultural hay meadow with areas delimited by hedges, tree lines or canals, forest and heather, and mere and marsh in the depressions. The brook valleys have a high ecological value and include moist Franguletea on nutrient-rich soil, nitrophilic alluvial alder forest, reed vegetation and marsh marigold fields. This area is also very attractive to a wide range of bird species due to the diversity of vegetation types.
The high water levels and regularly recurring floods of the Laak make flood areas inaccessible. Due to the diversity of flora and fauna, this area is of great ecological importance and the quality of the surface water running through it must be protected.