By Svante Strandberg
In: Ortnamnssällskapets i Uppsala årsskrift 2019
From Lake Oppäsen in Fellingsbro parish, Västmanland, a river flows into Lake Finnåkerssjön and from there into the river Arbogaån. The stretch of the river between Finnåkerssjön and the Arbogaån is called Ässingån (esnån 1754). Running alongside it is a pronounced gravel ridge, or esker. There is also an esker by part of Lake Oppäsen. The author is primarily inclined to assume that, in early times, the Ässingån had a name in *Æsn-, meaning ‘the ridge river, the river by the ridge’, and that the form –äsen in Oppäsen is a lake name formed from the name of the river.