Altrhein Kleiner Bodensee, Protected nature area in Karlsruhe, Germany.

Altrhein Kleiner Bodensee is a protected nature reserve near Karlsruhe encompassing an old river channel, wetland areas, and multiple forest types in close proximity to the Rhine. The site combines oxbow lakes, marshy zones, and woodland habitats within a cohesive landscape.

The old Rhine arm known as Kleiner Bodensee was cut off from the river and now has no direct connection to it, covering 300,000 m² of water. The area was designated a nature reserve by regulation on 13 June 1985 under protection number 2.081, and it later became part of the Natura 2000 site "Rheinniederung von Karlsruhe bis Philippsburg" through the EU LIFE project "Lebendige Rheinauen bei Karlsruhe".

The name Kleiner Bodensee, "little Lake Constance", is a local nickname for this oxbow of the Rhine. The reserve lies in the Rhine floodplain where the Alb flows into it; its lower course and part of the Alb canal cross the protected area. The site is part of the EU LIFE project "Lebendige Rheinauen bei Karlsruhe".

The area is accessible through multiple hiking paths offering routes of different lengths and difficulty levels. Visitors should wear sturdy footwear and bring insect protection, especially during wetter months.

In summer the old Rhine arm is covered by leaf rosettes of the water chestnut, a rare plant in this region; the reserve holds over 500 plant and 130 bird species. The water chestnut's floating leaves form a dense layer on the 300,000 m² water surface, which has no direct connection to the river today.