ASBOG Drainage Patterns/Stream Types 2 — Questions and Answers
Question 1: What is a trellis drainage pattern, and what does it indicate about the underlying geology?
- A drainage pattern with streams flowing radially outward from a central high point, indicating a dome or volcano
- A drainage pattern with long parallel main streams joined at right angles by short tributaries, indicating alternating ridges and valleys of tilted, folded, or faulted rock (Correct answer)
- A drainage pattern with randomly branching streams, indicating a horizontal, homogeneous substrate
- A drainage pattern with streams converging toward a central depression, indicating a basin or karst feature
Correct answer: A drainage pattern with long parallel main streams joined at right angles by short tributaries, indicating alternating ridges and valleys of tilted, folded, or faulted rock
Trellis drainage develops on alternating bands of resistant and weak tilted rock (such as folded sedimentary sequences), producing long main streams in weak rock valleys connected at right angles by short tributaries crossing the resistant ridges.
Trellis drainage patterns are strongly controlled by rock structure. They develop in fold-and-thrust belts, cuesta landscapes, and tilted sedimentary sequences where alternating resistant (sandstone, quartzite) and weak (shale, limestone) rock bands create alternating ridges and valleys. Main streams (subsequent streams) occupy the weak rock valleys, while tributaries (obsequent and resequent streams) join them at nearly right angles after crossing the resistant ridges. The Appalachian Valley and Ridge province is a classic example of trellis drainage.
Question 2: In the Strahler stream ordering system, how is the order of a stream reach determined?
- All streams are assigned order 1 at the headwaters, and order increases by 1 each time two streams of equal order join; the junction of unequal orders retains the higher order (Correct answer)
- Stream order is assigned by the US Geological Survey based on total catchment area and length
- Each tributary downstream junction increases stream order by 1 regardless of the tributary's order
- Stream order is determined by the gradient — steeper streams have higher order numbers
Correct answer: All streams are assigned order 1 at the headwaters, and order increases by 1 each time two streams of equal order join; the junction of unequal orders retains the higher order
In the Strahler system, unbranched headwater streams are order 1; when two order-1 streams join, they form order 2; when two streams of the same order n join, they form order n+1; junction of streams of different orders retains the higher order.
The Strahler stream ordering system (1952) is the most widely used scheme for quantifying drainage network complexity. Rules: (1) all headwater (fingertip) tributaries with no tributaries of their own are order 1; (2) when two streams of order n join, they create a stream of order n+1; (3) when two streams of different orders join, the downstream segment retains the higher order. The Horton-Strahler number of the main stream equals the order of the entire basin. Stream order correlates with discharge, channel width, depth, and gradient, and is used in geomorphic analysis, ecological classification, and flood hazard assessment.
Question 3: What is an antecedent stream, and how does its drainage pattern provide evidence for tectonic uplift history?
- A stream that follows a path predetermined by the regional slope of a newly emerged coastal plain; evidence of sea level fall
- A stream that predates tectonic uplift and maintains its course by eroding downward as a barrier rises across its path, creating a water gap or gorge through an otherwise wind-gap ridge (Correct answer)
- A stream that captures drainage from an adjacent basin by headward erosion; evidence of differential erosion rates
- A stream flowing in the opposite direction from regional drainage due to thrust faulting; evidence of tectonic inversion
Correct answer: A stream that predates tectonic uplift and maintains its course by eroding downward as a barrier rises across its path, creating a water gap or gorge through an otherwise wind-gap ridge
An antecedent stream existed before tectonic uplift and maintained its original course by incising downward at a rate equal to or greater than the rate of uplift, cutting a gorge or water gap through the rising structure.
Antecedent drainage is evidence that erosion rates matched or exceeded uplift rates during tectonic deformation. Classic examples include the Indus and Brahmaputra rivers, which cut spectacular gorges through the Himalayas — these rivers existed before the mountain building and maintained their courses as the mountains rose around them. The Delaware Water Gap in the Appalachians is another example. Antecedent streams differ from superimposed streams (streams let down through unconformable cover onto underlying structure) and from consequent streams (streams whose course is determined by initial topographic slope).
Question 4: What is the primary difference between a braided river and an anastomosing river system?
- Braided rivers have multiple unstable, shifting channels separated by transient bars with high width-to-depth ratios; anastomosing rivers have multiple stable, vegetated channels with low width-to-depth ratios separated by floodplain islands (Correct answer)
- Braided rivers occur only in arid climates; anastomosing rivers only in humid climates
- Braided rivers have higher sinuosity than anastomosing rivers; both transport bed load and suspended load
- Braided rivers form only in mountain headwaters; anastomosing rivers only in coastal deltas
Correct answer: Braided rivers have multiple unstable, shifting channels separated by transient bars with high width-to-depth ratios; anastomosing rivers have multiple stable, vegetated channels with low width-to-depth ratios separated by floodplain islands
Braided channels are wide, shallow, shifting, and separated by unstable mid-channel bars — typical of high sediment load and flashy discharge; anastomosing rivers have multiple stable channels separated by vegetated floodplain islands with low width-to-depth ratios and finer, cohesive banks.
Braided rivers (high-energy, high sediment load, steep gradient, non-cohesive banks) form multiple, interweaving channels separated by transient sand and gravel bars that shift position with each flood. They are common in pro-glacial environments, alluvial fans, and high-gradient mountain rivers. Anastomosing rivers (low-energy, low gradient, cohesive organic-rich banks) maintain multiple stable channels separated by semi-permanent vegetated islands and have low width-to-depth ratios. They are common in boreal and humid wetland environments. Channel stability in anastomosing rivers is maintained by strong, vegetated banks resistant to lateral migration.
Question 5: What is the geomorphic significance of a 'yazoo stream' on a river floodplain?
- A yazoo tributary runs parallel to the main river on the floodplain because natural levees block its direct entry into the main channel, forcing it to travel long distances to find a low point to join (Correct answer)
- A yazoo stream is a cutoff meander lake (oxbow lake) formed when the main channel cuts through a meander neck
- A yazoo stream is a tributary that captures the main river by stream piracy during a flood event
- A yazoo stream is the lowest-order headwater stream in a drainage basin, flowing from a spring
Correct answer: A yazoo tributary runs parallel to the main river on the floodplain because natural levees block its direct entry into the main channel, forcing it to travel long distances to find a low point to join
A yazoo tributary runs parallel to and alongside a major river for long distances on the floodplain because the main channel's natural levees are high enough to prevent the tributary from joining the main river until a low point in the levee system is found.
The Yazoo River in Mississippi (from which the term derives) is the type example: it flows parallel to the Mississippi River for over 100 km before finding a low enough gap in the Mississippi's natural levees to join it. Natural levees are built up during floods as coarse sediment is deposited immediately adjacent to the channel, creating elevated ridges. Tributaries entering the floodplain behind these levees cannot directly enter the main channel and must travel parallel to it until they find a confluence point. Yazoo tributaries are characteristic features of large, well-developed meandering river systems.
Question 6: What is the Hjulström-Sundborg diagram used for in fluvial geomorphology, and how does it relate to stream power?
- It plots stream gradient against discharge to classify rivers into braided, meandering, and straight channel types
- It relates current velocity (or stream power) to grain size to predict erosion, transportation, and deposition thresholds for different sediment types, including the critical velocity for erosion of different grain sizes (Correct answer)
- It shows the relationship between river sinuosity and valley gradient for different climate regimes
- It predicts floodplain width as a function of drainage area and mean annual precipitation
Correct answer: It relates current velocity (or stream power) to grain size to predict erosion, transportation, and deposition thresholds for different sediment types, including the critical velocity for erosion of different grain sizes
The Hjulström (and Sundborg's revision) diagram plots mean current velocity versus sediment grain size, defining fields where erosion, transportation, and deposition occur, demonstrating that cohesive fine sediments require higher critical velocities for erosion than medium sands.
The Hjulström-Sundborg diagram is a fundamental tool in fluvial sedimentology for predicting sediment behavior under different flow velocities. The erosion threshold (critical velocity) curve shows that medium sand (~0.2–0.5 mm) requires the lowest critical velocity (~20 cm/s) for entrainment, while both coarser particles (require more force to overcome gravity) and finer cohesive sediments (require more force to overcome electrostatic bonds) need higher critical velocities. The settling velocity curve defines the conditions for deposition. Stream power (ω = ρgQS, where Q is discharge and S is slope) is related to velocity and determines whether a stream is erosional, transportational, or depositional in a given reach.
ASBOG Drainage Patterns/Stream Types 2
What is a trellis drainage pattern, and what does it indicate about the underlying geology?