Across the United States and even within Arizona, streams exhibit great variability. Understanding the differences among stream types and the ways they can be classified is important for several reasons, including:

• Enhancing communication between stream managers, researchers, and the public by providing a framework
• Describing existing conditions and trends in stream characteristics
• Enabling prediction of stream behavior from its appearance
• Development of hydrologic and sediment relationships
• Comparison between streams with similar geomorphic characteristics
• Extrapolation of data from a well-studied stream to other streams with similar characteristics

Many ways of classifying stream exist, based on either physical characteristics, biological characteristics, or a combination of both. We will cover only the most commonly encountered classification systems here.