the Grand Canyon area from being semi-arid. The uplift of the Colorado Plateau is uneven, and the Kaibab Plateau that the Grand Canyon bisects is over higher at the North Rim than at the South Rim. Almost all runoff from the North Rim (which also gets more rain and snow) flows toward the Grand Canyon, while much of the runoff on the plateau behind the South Rim flows away from the canyon (following the general tilt). The result is deeper and longer tributary washes and canyons on the north side and shorter and steeper side canyons on the south side. Temperatures on the North Rim are generally lower than those on the South Rim because of the greater elevation (averaging above sea level). Heavy rains are common on both rims during the summer months. Access to the North Rim via the primary route leading to the canyon (State Route 67) is limited during the winter season due to road closures. Geology The Grand Canyon is part of the Colorado River basin which has developed over the past 70 million years. Over more than 150 years, scientists have gathered data, proposed new ideas, and debated sometimes contentious theories about the geologic origins of the Grand Canyon and the Colorado River. Formation of the Grand Canyon and the Colorado River may involve a complex history in which multiple factors and geologic processes have interacted over time and in different locations. In the most recent round of “old river” vs. “young river” controversy, researchers have challenged estimates that had placed the age of the canyon at 5–6 million years. The research has aroused considerable controversy because it suggests a substantial departure from prior widely supported scientific consensus. In a 2008 study, Victor Polyak examined caves near the Grand Canyon and placed their origins about 17 million years ago. The study, which was published in the journal Science in 2008, used uranium-lead dating to analyze calcite deposits found on the walls of nine caves throughout the canyon. In another 2008 study, Rebecca Flowers reported on apatite (U-Th)/He thermochronometry results suggesting that parts of the Grand Canyon had reached a depth near to the modern depth around 20 million years ago. In a subsequent study published in the journal Science in 2012, she suggested that the western part of the Grand Canyon could be as old as 70 million years. The emerging scientific consensus is