Stream order and drainage patterns help students describe how water moves across Earth’s surface. This reference covers how small streams join to form larger streams and how stream networks reveal clues about slope, rock type, structure, and landscape history. Students need these ideas to interpret topographic maps, watershed diagrams, aerial photos, and real drainage basin data.
The core method is the Strahler stream order system, where the smallest unbranched channels are first order streams. When two streams of the same order meet, the order increases by one, but when different orders meet, the higher order continues downstream. Drainage patterns such as dendritic, trellis, radial, rectangular, and parallel form because water follows gravity while being guided by geology and landform shape.
Key Facts
- In the Strahler system, the smallest unbranched stream channel is assigned stream order 1.
- When two streams of the same order join, the downstream segment increases by one order, such as 2 + 2 = 3.
- When two streams of different orders join, the downstream segment keeps the higher order, such as 1 + 3 = 3.
- A tributary is a smaller stream or river that flows into a larger stream or river.
- A drainage basin, or watershed, is the land area where all runoff drains to the same main stream, river, lake, or ocean outlet.
- A drainage divide is a ridge or high area that separates one drainage basin from another.
- A dendritic drainage pattern looks like branching tree limbs and usually forms on fairly uniform rock or sediment.
- A radial drainage pattern flows outward from a central high point, such as a volcano, dome, or isolated mountain.
Vocabulary
- Stream order
- Stream order is a numbering system that ranks stream segments within a drainage network by their position and branching pattern.
- Tributary
- A tributary is a smaller stream that flows into a larger stream or river.
- Drainage basin
- A drainage basin is the total land area drained by a stream system to one common outlet.
- Drainage divide
- A drainage divide is a high boundary that separates neighboring drainage basins.
- Dendritic pattern
- A dendritic pattern is a branching drainage pattern that resembles a tree and forms where rock resistance is fairly uniform.
- Trellis pattern
- A trellis pattern is a drainage pattern with nearly parallel main streams and short tributaries that join at close to right angles.
Common Mistakes to Avoid
- Adding stream orders every time streams meet is wrong because only two streams of the same order increase the downstream order by one.
- Changing a stream from order 3 to order 4 when a first order tributary enters is wrong because 1 + 3 remains order 3 in the Strahler system.
- Labeling all branching drainage networks as dendritic is wrong because trellis, rectangular, and parallel patterns also branch but reflect different slopes and rock structures.
- Confusing a drainage divide with a riverbank is wrong because a divide is a high boundary between basins, not the edge of a channel.
- Assuming drainage patterns are random is wrong because they are controlled by slope, rock resistance, faults, joints, folds, and landform shape.
Practice Questions
- 1 Two first order streams join to form a new stream segment. What is the order of the new segment?
- 2 A third order stream is joined by a second order tributary. What is the order downstream from the junction?
- 3 In a drainage network, two second order streams join, and the resulting stream later joins a third order stream. What is the final downstream order after both junctions?
- 4 A map shows streams flowing outward in all directions from a central mountain peak. Identify the drainage pattern and explain what it suggests about the landscape.