Tulare Lake was a large, shallow lake in eastern Amador Valley, surrounded by Willow Marsh (also known as the Lagoon). Tule rushes and willow trees once lined the marshes and sloughs of its shores. Drainage alterations starting in the 19th century have since reduced the marsh to the Arroyo de la Laguna, and the city of Pleasanton has since expanded across what was once marshland. Such rapid developments have led to tricky seasonal flow variations in Niles Canyon.