Data on sediment yield for drainages over various time frames.

 

The data below was takden from the data repository Appendix for article Kirchner, J. W. and others, 2001, Mountain erosion over 10 yr, 10 k.y. and 10 m.y. time scales; Geology. vol. 29, p 591-594.

Whenever a rate is determined, in this case sediment yield for a drainage basin, it is computed for a given time span over which change has been measured. The question can be asked - how do rates scale with this record length? Do they vary as a function record length? The clear answer for many geomorphic rates is that they do scale with record length. You can think of short term vs. long term rates and how they might differ.

This phenomena is very important to be aware of. If you use a short term rate and extrapolate to a longer time period, or do the opposite, you will get the wrong answer if the rates scale with record length. The below data allows one to gain some initial insight into these questions. A major conclusion is that short term rates are an order of magnitude less than long term rates (assuming that the two different methods of rate calculation are accurate). That can be easily seen below by just looking at the numbers. Units for long term sediment yield are the same as the short term.- tones per square km per year.

                         
            drainage area km2 altitude max altitude min record length in yrs short term sediment yield T.km-2.yr-1 time scale long term sediment yield
SC- 2 44º 22'20" 115º 45'59" 1.2 1463- 2073 27 13.2 5100 327
SC- 3 44º 22'06" 115º 46'12" 1.3 1451- 2066 28 8.9 9400 174
SC- 5 44º 20'46" 115º 47'18" 1.1 1390- 1772 28 10.9 12000 136
SC- 6 44º 20'15" 115º 48'20" 1.6 1381- 1784 27 9.3 11000 152
SC- 7 44º 21'01" 115º 46'30" 0.23 1457- 1819 22 14.4 17000 90
SC- 8 44º 21'21" 115º 47'02" 1.1 1415- 1720 13 30 13000 121
HC- 2 45º 59'37" 115º 20'27" 0.57 1268- 1744 10 7.3 16000 97
HC- 4 45º 59'44" 115º 20'39" 1.4 1280- 1747 10 3.5 18000 89
HC- 6 45º 59'37" 115º 21'00" 1 1293- 1726 15 3.3 19000 80
HC- 8 45º 59'41" 115º 21'26" 1.5 1317- 1726 12 11 17000 90
HC- 9 45º 59'37" 115º 21'53" 0.23 1341- 1646 10 8.6 19000 80
HC- 10 45º 59'32" 115º 22'19" 0.65 1354- 1729 12 9.9 17000 92
HC- 12 45º 59'30" 115º 23'07" 0.83 1390- 1804 14 8.2 16000 101
HC- 14 45º 59'24" 115º 23'30" 0.62 1415- 1804 12 7.5 19000 80
HC- 16 45º 59'27" 115º 24'31" 0.21 1524- 1768 13 25.1 27000 55
West Fork 45º 59'30" 115º 20'10" 17 1250- 1804 23 5 18000 87
East Fork 45º 59'20" 115º 19'58" 14 1241- 1835 23 2.5 20000 76
Tailholt A 45º 03'11" 115º 40'54" 2.2 1256- 2369 21 11 6300 264
Tailholt B 45º 03'10" 115º 40'57" 1.6 1256- 2141 22 14.6 6400 262
Tailholt C 45º 03'09" 115º 41'00" 1.4 1256- 2073 22 13.7 8200 202
Tailholt Main 45º 02'34" 115º 40'38" 6.6 1091- 2369 28 14 7000 239
Circle End 45º 03'17" 115º 40'13" 0.8 1296- 2015   N. 7300 226
Circle End 45º 03'16" 115º 40'17" 2.3 1296- 2369   N. 7300 229
Circle End 45º 02'50" 115º 40'01" 8 1083- 2369 25 6.5 7700 215
Trapper Creek 45º 40'13" 115º 19'47" 20 1476- 2012 10 9.8 26000 57
South Fk. 45º 42'35" 115º 20'37" 98 1320- 2170 14 8 25000 58
Upper Red 45º 42'38" 115º 20'34" 129 1320- 2075 14 10.1 18000 87
Johns Creek 45º 49'23" 115º 53'18" 293 735- 2551 10 7.6 15000 108
S. Fk. 45º 53'15" 116º 01'47" 2149 600- 2551 25 7.6 17000 91
Lochsa River 46º 09'04" 115º 35'37" 3055 465- 2680 72 26.3 6700 250
Selway River 46º 05'11" 115º 30'59" 4945 490- 2709 70 24.5 8100 205
Salmon River 45º 44'38" 116º 19'39" 35079 460- 3859 84 13.7 6300 261

Looking 'inside' the short term rates there is evidence of a 'poor' relationship consistent with the major conclusion that rate increases with record length it is computed over. The first regression is if one forces the best fit line to go through 0. The second line is the best fit without such a restriction. Which one makes more sense geologically. Using the second line,if one were to extrapolate to a 1000 year record length the sediment yield would be circa 200, broadly similar to the average for the long term rates.

 

An interesting implication of the above plot is that for this method of estimation the longer the duration of record the lower the sediment yield, opposite to the conclusion reached when comparing recent short term events and longer term estimates. A general relationship has been established for this scaling ( Gardner & others, 1987) The relationship is also fairly well constrained with the r-squared value of .9669. However, look at the graph. Something indicates the fit could be improved with a different form of equation. What is it about the relationship between the points plotted and the best-fit curve computed that indicates it could be improved?

Remember, there may be multiple reasons for this relationship, including some aspect of the technique of estimating sediment yield from cosmogenic nuclide concentrations in the alluvial sediment.

Note that you can look for other relationships in the above data.


Gardner & others, 1987, Geomorphic and tectonic process rates: effects of measured time interval: Geology, 15 259-261.