LONG-TERM EROSION RATES OF COASTAL WATERSHEDS IN THE PACIFIC NORTHWEST

We measured the Be-10 concentration of alluvial sediment leaving 16 coastal
watersheds in Northern California and Oregon to determine mean basin-wide
erosion rates of the Northwest Coast Ranges. The large spatial scale of these
watersheds allows us to relate erosion rates to uplift rates between latitudes
39 degrees and 47 degrees north. Erosion rates are highest in Northern
California, approaching 0.8 mm/yr in the Van Duzen River watershed, and reach a
low of less than 0.1 mm/yr in the Central Oregon Coast Range, but increase again
in the northern part of the Oregon Coast  Range (see graph below). Our data
confirm previous suggestions that erosion rates balance rock uplift in the
Central Oregon Coast Range.  To the south, near the Mendocino Triple Junction,
our basin-scale erosion rate estimates of 0.3 - 0.8 mm/yr are lower than
estimated uplift rates of 1 mm/yr. This suggests that rock uplift may be
outpacing erosion, leading to an increase in the average elevation of the coast
range, but the comparison is complicated by differences in the spatial scale and
averaging time of the uplift and erosion rate measurements.


------------------------------------------------- 1.0
|                                               | 0.9
|     o                                         | 0.8
|                                               | 0.7   E
|                                               | 0.6
|   o                                           | 0.5 (mm/
|        o                                      | 0.4   yr)
|      o    o                              o    | 0.3
|                   o                     o     | 0.2
|                       o    oooo               | 0.1
------------------------------------------------- 0
|     |     |     |     |     |     |     |     |
39    40    41    42    43    44    45    46    47
                   N Latitude