It is a left tributary (east) of the Lahn in the Vorderer Vogelsberg region.

It rises in Saasen and after 24.3 kilometres, flows into the Lahn at Giessen, near the railway station.

The saprobic index in the middle reaches of the Wieseck (at Großen-Buseck) is 1.77 which is considered "good" according to the WFD standards.

Several other indices calculated by evaluating the colonization of the Wieseck with aquatic organisms (macrobenthos), confirm this poor condition.

These values, which indicate structural impairment of the river, show that the middle reaches of the Wieseck at Großen-Buseck are "unsatisfactory", and the lower reaches in the urban area are "poor".