Seeache, Mountain brook in Achenkirch, Austria.

The Seeache is a mountain brook in Achenkirch that originates from Lake Achensee and carries crystal-clear water through the valley. The stream flows for several kilometers and forms a natural part of the landscape structure with a visible course from the lake to the lower valley.

The stream was first recorded in 1002 as Walchenesbah. Since the Achensee power station was built in the Inn valley, much of Lake Achensee's water has been diverted into the Inn rather than flowing down the Seeache. The natural mouth of the Walchen, as the lower course is called, has lain under the Sylvenstein reservoir since the late 1950s.

The stream is called Seeache in Austria and Walchen in Germany; Bavarian maps label the Tyrolean stretch Achenbach. It was first recorded in 1002 as Walchenesbah, a name of uncertain origin possibly from an old personal name. The Achenbach tributary joins the lower lake near Achenkirch's centre.

The brook is accessible from marked hiking trails that run around Achenkirch and offer several entry points. Parking areas are distributed nearby, and the brook can be reached on foot from various locations.

The stream changes its name along its course: in Austria it is called Seeache or Ache, but from the Pittenbach and Hühnerbach, which mark the border between Tyrol and Bavaria, it becomes the Walchen. Its natural mouth into the Isar has been submerged in the Sylvenstein reservoir since the late 1950s.