Stream Piracy on the Palouse River

Looking at the path of the Palouse River today, we might find it hard to imagine the river didn’t always flow down its current path, but instead was diverted some 15,000 years ago by a series of at least 40 catastrophic floods known today as the Missoula Floods.

Stream Piracy occurs when a stream or river drainage system or watershed is diverted from its own bed, when it flows instead down the bed of a neighboring stream (McKnight, 2005). This is what happened with the Palouse River, but instead of flowing down the bed of a neighboring stream (in Washtucna Coulee) it was captured by fractures that had been eroded away by the catastrophic event called the Missoula Floods.

Today the Palouse River flows southwest, making an abrupt 90° turn to the south and emptying into the Snake River. Before the Missoula Floods, it flowed through Washtucna Coulee and into the Columbia River. At that time a thick mantle of windblown Palouse soil separated the Palouse River from the Snake River (Trimble, 1950; Alt, 2001).

When the flood waters roared through, they stripped away the thick mantle of Palouse soil and exposed basalt that many millions of years ago had been weakened by a network of fractures, due to tectonic activity (Bjornstad, 2006). These narrow zones of fractured rock were more susceptible to erosion and the Missoula floods carved out a deep channel that was lower than Washtucna Coulee, which allowed the Snake River to capture the Palouse River permanently and form the beautiful landscape we enjoy today.

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