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  • What happens when a third-order stream connects with a second-order stream?

    Question

    what happens if a third-order stream and second-order stream connect? im asking for frank. hes being very annoying about the whole situation.

    Naturalist's Response

    Converging streams

    Hah! I’m always happy to “muddy the waters” of a disagreement. (See what I did there?)

    Unlike the simple rules of the Thunderdome (“Two men enter, one man leaves!”), there’s no single rule for labeling streams.

    Everybody agrees that a stream with NO tributaries is a “first-order stream.” And most people agree that when two first-order streams merge, they deserve to be called a “second-order stream.”

    That’s about where the agreement stops.

    There are two different methods whipped up in the 1950s and 1960s that are still in use today for ordering streams:

    The Strahler Method: Developed by Arthur Strahler, this method says that stream order only increases when two streams of the same order merge. If a lesser stream joins a greater stream, then the stream order doesn’t change. (For example, two second-order streams merge to make a third-order stream; but if a second-order stream joins a third-order stream, it remains a third-order stream.)

    Strahler method

    The Shreve Method: Developed by Ronald Shreve, this method takes the approach of adding the order numbers of any streams that merge. Every time there’s a merger, the order number changes. (For example, if a second-order stream joined a third-order stream, the result would be a fifth-order stream.)

    Shreve method

    There’s no single, correct method. Lots of hydrology planners prefer the Shreve method; it seems to be the more popular method as of 2021.

    However, the Shreve method has the advantage of telling you how many upstream channels there are at a glance; some watershed-based maps prefer this method.

    Regardless, the folks in the GIS and mapping industry are ready to switch between methods when necessary. For example, the ESRI company has a switch to let you choose which method to use for your map.

    (The streams, presumably, don’t care what humans label them; they just flow.)

    -Naturalist Chris Mentrek