Perhaps only three or four hundred years ago, an enormous rockfall dumped boulders across this canyon, damming Tenaya Creek. During spring and early summer, the stream backs up into the two pools on either side of the dam.
Tinkering with nature
Nineteenth-century tourists admired the reflection on the surface of the upper pool of Mirror Lake. To enlarge the pool and reflection, early entrepreneurs piled boulders onto the natural dam. Ironically, this helped the pool to fill with sand and silt, leading to regular dredging of the lake. Today, the National Park Service manages Tenaya Creek and Mirror Lake as a natural system.
Sediment continues to build in the upper pool, which is gradually becoming shallower. The force of springtime water scours out most deposits in the undammed lower pool.