The Strahler Stream Order System ranks streams based on the number of tributaries that have merged. It is a top-down system where rivers of the first order are the headwaters (aka outermost tributaries). Stream order rules: Headwaters are first-order streams. Higher-order streams are second-order, third-order, etc. Downstream segments are defined at tributary confluences. If two streams are not the same order at a confluence, the higher order is maintained for the downstream segment. If two streams are the same order at a confluence, the downstream segments is assigned the next- higher order. [1] 3 2 3 3 3 2 3 Figure 2: A simplified model of a river basin 5 5 3 2 Use Figure 2 to match each letter to its correct Strahler stream order rank. A B D E F G Strahler stream order rank #1 [Choose ] [Choose] Strahler stream order rank #1 [Choose ] [Choose] [Choose ] + + + + +