Wear of Unsound Pebbles in River Headwaters

Pebbles that are initially weathered, inhomogeneous, angular, or fractured ("unsound") become sound with transport. The Sternberg law describes well the wear of sound pebbles found in large rivers, but describes poorly that of unsound pebbles in river headwaters. For unsound pebbles the Sternberg coefficient (which is assumed to be a constant) decreases appreciably downstream. An alternative to the Sternberg law is derived in which the coefficient is proportional to the reciprocal of the downstream distance traveled. The laws are compared by using data from the Clutha basin in New Zealand.