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

Google Earth image

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)