One of the most common features in uvial environments is the systematic downstream de-
cline in grain size, which is usually attributed to either abrasion - the reduction in sediment
size due to attrition of mass - or selective sorting - the size segregation of grains due to their
relative transport mobility. Despite the ubiquity of this grain pattern and the extensive
research on both of these processes, there remains questions regarding the underlying prin-
ciples driving abrasion and sorting, as well as the relative contribution of these processes
to grain ning. Therefore, a mechanistic understanding of these processes is necessary to
observe their direct eect on pattern formation. This dissertation investigates the controls
and limits on abrasion and sorting through eld studies and laboratory experiments. First,
using the well-dened boundary conditions of an alluvial fan, we examine how grain hiding
limits gravel sorting by tracking changes in the grain size distribution measured using a
novel image-based technique. Further downfan, we compare surface sand fractions mea-
sured in the eld with those from the lab and show that the gravel-sand sorting proles
are self-similar, suggesting generality in their development. In a second eld study, using
detailed hand and image-based measurements characterizing size and shape of thousands of
grains throughout a watershed, we are able to directly observe the eectiveness of abrasion.
We then input these measurements into a simple numerical model to tease apart the con-
tribution of abrasion and sorting to downstream grains size and shape evolution. Finally,
we conduct laboratory experiments to isolate the eects of impact energy on abrasion rates
and use material properties of the grains to collapse mass loss curves between dierent
lithologies. We measure the grain size distribution of the products of abrasion to show
that they are in agreement with expectations from brittle fracture theory. The results from
this work indicate that both sorting and abrasion are eective mechanisms in producing
downstream grain size patterns. Because grain size exerts a strong control on channel mor-
phology, understanding the controls on particle size change fosters a more complete picture
of the uvial system.
Litwin Miller, Kimberly; Jerolmack, Douglas (2014): The Causes and Consequences of Particle Size Change in Fluvial Systems. University of Pennsylvania.
This Paper/Book acknowledges NSF CZO grant support.
(106 MB pdf)
The Causes and Consequences of Particle Size Change in Fluvial Systems