Which river systems are known to supply Maricopa County?

The Salt River and the Gila River are the primary river systems that supply water to Maricopa County. The Salt River provides a significant amount of water for agriculture and urban use in the area. It is part of the larger Colorado River system, which is crucial for the water supply across the southwestern United States. The inclusion of the Colorado River in the correct choice underscores its importance as a vital water resource that also supports the Salt and Gila rivers.

In contrast, the other options include rivers that are either geographically distant from Maricopa County or are not significant sources of water for the region. The Rio Grande and the Verde River, although they may have relevance in other contexts, do not serve the water supply needs of Maricopa County in the same way that the Salt and Gila rivers do. The Columbia and Missouri rivers, while major waterways in the United States, are not connected to the water supply of Maricopa County at all. Thus, option A accurately reflects the local river systems essential to the area.