Long ago, the Colorado River flowed through country so flat that its course constantly changed as silt deposits blocked its flow. At times, the river flowed into the Gulf of California, while at other times it headed west into a low lying area called the Salton Sink. Each time the sink filled with river water, a large lake formed. This lake was called Lake Cahuilla. It covered over 100 miles, stretching from Palm Springs southward into Mexico.
Each time the river changed its course from the sink to the Gulf of California, the lake dried up, leaving behind miles of sandy beaches. Northwesterly winds blew across the dry lake bed and swept the sandy beaches to the southeast. Eventually, the sand piled up on itself and formed dunes.
What stops the dunes from blowing further south?