# Mountain Front Sinuosity

Contents
1.) Mountain Front Sinuosity
2.) Drainage Outlet Spacing

Mountain Front Sinuosity
Straight mountain fronts tend lie along active faults. Mountain Front Sinuosity is a classic index of tectonic activity.

Smf = Lmf / Ls

Smf = Mountain front sinuosity
Lmf = Sinuous length measured along an undulating, weaving path at the mountain hillslope-alluvial fan slope break.
Ls = Straight-line length of the mountain front segmentÂ

Use the Measure tool to draw a path (multiple vertices) between two defined end points that follows the slope break between alluvial fans and fault-controlled hillslope (faceted faces). Right-click to select line > Properties to get its length. Use the same tool to determine the straight-line distance between the same two points.

Refs:
Bull and McFadden (1977), Bull (1978), Keller (1986)

Drainage Outlet Spacing
Hovius (1996) suggested that spacing between drainage outlets along linear mountain fronts is approximately equal to half the perpendicular distance between the mountain front and the main drainage divide, the Himalayas excluded. Its an easy model to test.

Pick a linear range front (or a few of them) and collect some data. Consider structurally/topographically coherent portions of mountain ranges. Use the Measure tool to determine the straight-line distances between drainage divide and range front at the point where stream exits the mountains. You’ll have two ridge-to-range front measurements for every one drainage-to-drainage measurement. Chart either/or the ratio of averaged values in an XY scatter plot with model trendline for each rangefront (one dot for each rangefront).

Hovius’ model:
Sd = (DIV-MF)/2

Sd = Drainage spacing
DIV = Drainage divide
MF = Mountain front

Example:
MF to RL: 1590+1575+1815+1740+1870 = 3680/4 = 920*2 = 1840m
D to D: 1050+845+800+985 = 8590/5 = 1718m

QUESTIONS:

Does the model have merit?
How would you further test it (beyond example)?
How does the morphology of the range front or ridgeline affect the model?

Refs:
Perron et al. (2009), Hovius (1996), Talling et al. (1997), Shaler (1899), Tarboton et al. (1992)