Final answer:
The precipitation decreases to the west of the Colorado River due to the rain shadow effect caused by the Andes Mountains.
Step-by-step explanation:
The question is asking about the cardinal direction in which precipitation decreases north of the Colorado River. The given information states that the Andes Mountains cause a rain shadow effect, where moisture from rain clouds precipitates on the eastern slopes, resulting in little or no precipitation on the western side of the mountains. Therefore, to the west of the Colorado River, the precipitation decreases, indicating a decrease in precipitation towards the west.