course has been modified several times throughout ancient geological history. Prior to 10 million years ago, the Kern River flowed into the San Joaquin Valley at a point further south, along what is now Walker Basin Creek, which outlets north of Arvin. Uplift west of the Kern Canyon Fault blocked the river and forced it to cut a new course further north, forming the steep gorge below Lake Isabella and Bakersfield. The upper part of the Kern River canyon, at least above Golden Trout Creek, was widened and deepened by glaciers during the Ice Ages. The Kern Canyon fault passes very close to Isabella Dam, and is considered a threat to the dam's structural stability. The Kern River Oil Field is adjacent to the river on the north, just before the river flows into Bakersfield. The large oil field, on low hills which rise gradually into the Sierra foothills, formerly allowed much of its wastewater to drain directly south into the river. However, modern environmental regulation ended this practice, and the contaminated water is now cleaned at water treatment plants and used to irrigate farms in the valley to the west. Discharge Due to water diversion and Isabella Dam the Kern River's discharge changes considerably over its length. The highest mean annual flows occur just downriver of Isabella Dam, but because the dam serves to regulate the flow of water the highest daily discharges occur above the dam on the North Fork section of the Kern River. The USGS stream gauge on the North Fork Kern River has recorded an average annual mean discharge of and a maximum daily discharge of , and the gauge on the South Fork Kern River shows an average annual mean discharge of and a maximum daily discharge of . In contrast the first stream gauge below Isabella Dam has recorded an average annual mean of but a maximum daily discharge of only . Due to water withdrawals the three stream gauge stations below Isabella Dam show a dramatically decreasing discharge. At the last gauge, near Bakersfield, the river's average flow is only . Recreation Kern Canyon, the deep canyon of the river northeast of Bakersfield, is a popular location for fishing and boating, particularly fly fishing and whitewater rafting, whitewater kayaking, and riverboarding. Of particular interest to fishermen are the Kern River rainbow trout, the Little Kern golden trout, and the California golden trout.