The Ilse is a river that rises at about 900 m above sea level on the northern slopes of the Brocken. During its first few kilometres it flows as a narrow brook, almost invisible to the observer, down the side of the Brocken. Known here as the Verdeckte Ilse ("hidden Ilse") it gurgles its way under blocks of granite that hide the stream bed from above.

The Ilse then rushes through the narrow Ilse valley, hemmed in to the east by the rugged, cross-topped Ilsestein, passes by Ilsenburg

Ilsenburg

Ilsenburg is a town in the district of Harz, in Saxony-Anhalt in Germany. It is situated under the north foot of the Harz Mountains, at the entrance to the Ilsetal valley of the small Ilse river, a tributary of the Oker, about six north-west of the town of Wernigerode. It received town privileges...

 into the Harz Foreland, flows through Veckenstedt

Veckenstedt

Veckenstedt is a village and a former municipality in the district of Harz, in Saxony-Anhalt, Germany. Since 1 January 2010, it is part of the municipality Nordharz....

, Wasserleben

Wasserleben

Wasserleben is a village and a former municipality in the district of Harz, in Saxony-Anhalt, Germany. Since 1 January 2010, it is part of the municipality Nordharz....

, Osterwieck

Osterwieck

Osterwieck is a historic town in the Harz district, in the German state of Saxony-Anhalt. It is situated on the river Ilse, north of Wernigerode and the Harz mountain range. On 1 January 2010 the municipalities of the former Verwaltungsgemeinschaft Osterwieck-Fallstein merged in Osterwieck.The...

 and Hornburg

Hornburg

Hornburg is a town in the Wolfenbüttel district, in the German state of Lower Saxony. It is situated at the Ilse river, a tributary of the Oker. Hornburg is part of the Samtgemeinde Schladen and home to numerous historically valuable half-timber buildings...

 and discharges into the Oker

Oker

The Oker is a river in Lower Saxony, Germany, that has historically formed an important political boundary. It is a left tributary of the River Aller, in length and runs in a generally northerly direction.- Course :...

 near Börßum

Börßum

Börßum is a municipality in the district of Wolfenbüttel, in Lower Saxony, Germany.-Geography:Börßum lies only a few kilometers north of the Harz Mountains, north of Schladen, on the east bank of the Oker, 11 km south of Wolfenbüttel. It is not far southeast of the Oderwald....

 together with the Schiffgraben from the Großes Bruch

Großes Bruch

The Großes Bruch is a wetland area, 45 kilometres long, in Germany, situated between Oschersleben in Saxony-Anhalt in the east and Hornburg in the west. It was formed from a glacial valley...

. Its total length is around 40 kilometres (24.9 mi).

Local folklore personifies the river as the beautiful Princess Ilse, who has her home in the rocks of the Ilsestein.
The Ilse is mentioned in literary works such as the Harzreise ("Harz Journey") by Heinrich Heine

Heinrich Heine

Christian Johann Heinrich Heine was one of the most significant German poets of the 19th century. He was also a journalist, essayist, and literary critic. He is best known outside Germany for his early lyric poetry, which was set to music in the form of Lieder by composers such as Robert Schumann...

.