Fluvial Channel Branching Enforces Threshold Relief. Issue 3 (29th January 2021)

• Record Type: Journal Article
• Authors: Cunningham, Maxwell T.; Kaplan, Michael R.
• Is Part Of: Geophysical research letters. Volume 48:Issue 3(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-01-29
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL091464
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Relief within mountain belts appears to be limited to a threshold of 1, 000–1, 500 m. However, it remains unclear definitively where in the landscape this threshold is found. Here we use a new method of analysis, termed Progressive Hypsometry, to show that "threshold relief" is the typical relief of first‐order fluvial catchments. The method tracks changes in the elevation frequency distribution (hypsometry) across nested catchments of all sizes. We find that self‐similar nesting of fluvial catchments leads to quasi‐scale invariance in hypsometry, such that the relief between any catchment's modal and outlet elevation ( R Mo, mode relief) falls into three groups: the two lower associated with steep upland subcatchments ( R Mo = 250–1, 000 m) and the other with fluvial catchments, which span orders of magnitude in area but maintain an R Mo of 1, 000–1, 500 m. We conclude that threshold relief is the maximum relief a first‐order fluvial catchment can develop without branching. Plain Language Summary: From 30, 000 feet, every mountain range is unique: the pattern of tectonic collision, geologic structure, and the local climate interact to create topographic idiosyncrasies. However, all mountains contain similar topographic patterns. One of the most striking is a limit on the local relief, the maximum elevation difference observed within mountain ranges. Analyses of digital elevation models show that local relief around the world rarely exceeds a threshold of 1, 000–1, 500 m. Many researchers have attributed this relief threshold to the strength of bedrock hillslopes, which collapse when they steepen beyond ∼35°. Here, we use a novel method of topographic analysis to show that instead of hillslopes, first‐order fluvial catchments tend to make up the threshold relief. These catchments cannot grow beyond 1, 000–1, 500 m before river branching splits them in two. As a result, first‐order catchments tend to have a geometrically similar shape. By the same token, our analysis finds isolated catchments subject to long‐term changes in shape due to competition with neighboring catchments. Such competition results in areas of disequilibrium rising to high elevations as they are slowly invaded by their neighbors. The rate at which disequilibrium catchments are eroded away may play an important role in the growth of mountain ranges. Key Points: "Threshold relief" in mountainous landscapes is the typical relief of first‐order fluvial catchments Relief between catchment modal and outlet elevation is scale invariant where the scale of analysis exceeds that of first‐order catchments Hypsometry records transience in catchment geometry induced by drainage capture