Following The Path of Least Resistance By: Sara Jose – SWAT Crew Member
A drainage basin is an area of land where water from precipitation drains into a waterbody, like a river or lake. Drainage basins include the land the water drains off of, and the streams the water is carried through. A drainage pattern is a pattern that streams, or tributaries form within a drainage basin in the land. Drainage patterns are classified based on how they form, the local topography, and potential obstacles that can be encountered. Streams will follow the path of least resistance, meaning that the pattern formed is dependent on the topography, and the type of subsurface below. If the subsurface is made up of rocks with many fractures within, the drainage pattern will be similar to the fractures within the rock below. In areas with soil that is uniform and has no specific structure, the subsurface can be eroded in all directions easily, causing a variation in patterns. Included below are a list, diagrams, and description of basic drainage patterns.
Basic Drainage Patterns: 1. Dendritic Pattern A dendritic pattern looks like a branching pattern of tree roots within the land (Figure 1.). This pattern develops in areas where the geological subsurface of the area is mainly homogenous (same throughout). The subsurface in the area has similar resistance to weathering, resulting in no control over the directions that the tributaries take. Dendritic drainage patterns are the most common form of drainage pattern and can be found across Canada.
Figure 1: Dendritic Drainage Pattern Aerial Photo Credit: Sara Jose
2. Trellis Pattern Trellised drainage patterns appear similar to the common garden trellis (Figure 2.). These patterns form in areas where sedimentary rocks are folded or tilted. The sedimentary rocks are then eroded depending on the strength of the rocks. The Rocky Mountains located in British Columbia, and Alberta have many examples of trellis patterns found within.
Figure 2: Trellis Drainage Pattern Aerial. Photo Credit: Sara Jose
3. Radial Pattern Radial patterns form around isolated mountains, volcanoes, and hills (Figure 4.). These streams radiate outwards in all directions from a central high area (Figure 3.).
Figure 3: Radial Drainage Pattern Aerial Photo Credit: Sara Jose
Figure 4: Radial Drainage Pattern Profile. Photo Credit: Sara Jose
4. Centripetal Pattern Centripetal drainage is similar to radial drainage. However, instead of radiating outwards, the centripetal drainage flows inwards towards a central low area (Figure 6.). The individual streams of the centripetal drainage are often in a dendritic pattern flowing inwards toward a central low point (Figure 7.). This drainage often feeds into ephemeral lakes, which disappear in the warm seasons.
Figure 5: Centripetal Drainage Pattern Profile Photo Credit: Sara Jose Photo Credit: Sara Jose
Figure 6: Centripetal Drainage Pattern Aerial. Photo Credit: Sara Jose
5. Parallel Pattern Parallel drainage patterns form when there is a significant slope present. This pattern includes multiple streams that are parallel to each other, flowing in the same direction and following the slope of the surface (Figure 7.). The streams within this drainage pattern are often very swift, due to the lack of meandering present. Parallel patterns often occur in areas where significant slope is present.
Figure 7: Parallel Drainage Pattern Aerial Photo Credit: Sara Jose
6. Annular Pattern Annular drainage patterns are concentrically arranged streams following circular like paths (Figure 8.). These streams are often draining a basin where erosion has exposed the substructures. Bands of weak rock break a part in a ring like pattern.
Figure 8: Annular Drainage Pattern Aerial Photo Credit: Sara Jose
7. Rectangular Pattern Rectangular drainage patterns are common in areas where right-angled faults occur in the bedrock, like some types of granite bedrock (Figure 9.). They are common in areas with little change in topography with many fractures and faults. Rectangular drainage patterns rarely occur in Canada.
Figure 9: Rectangular Drainage Pattern Aerial Photo Credit: Sara Jose
8. Pinnate Pattern Pinnate drainage patterns appear as a tree-like pattern in the earth (Figure 10.). This drainage pattern often forms in uniform rock structures and fine-grained substrates.
Figure 10: Pinnate Drainage Pattern Aerial. Photo Credit: Sara Jose
9. Distributary Pattern Distributary drainage patterns are the diversions of channels and can form deltas. This pattern has one stream which diverts further into multiple streams (Figure 11.). Distributary drainage patterns are the opposite of dendritic drainage patterns.
Figure 11: Distributary Drainage Pattern Aerial. Photo Credit: Sara Jose
10. Deranged Pattern Deranged drainage patterns have no pattern to the development of rivers and lakes (Figure 12.). This pattern commonly occurs in areas with large amounts of limestone deposits. Surface water often disappears through Karst formations which are caverns and tunnels within limestone formations. The surface water drains into groundwater, and eventually reappears at the surface in another area. This type of pattern is common through eastern and central Canada.
Figure 12: Deranged Drainage Pattern Aerial. Photo Credit: Sara Jose
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