Clark, J. J., and P. R. Wilcock (2000), Effects of land-use change on
channel morphology in northeastern Puerto Rico, Geol. Soc. Am. Bull.,
112(12), 1763– 1777.
Abstract:
Between 1830 and 1950 much of northeastern
Puerto Rico was cleared for agriculture.
Runoff increased by 50% and
sediment supply to the river channels increased
by more than an order of magnitude.
Much of the land clearance extended
to steep valley slopes, resulting in widespread
gullying and landslides and a large
load of coarse sediments delivered to the
stream channels. A shift from agriculture
to industrial and residential land uses over
the past 50 yr has maintained the elevated
runoff while sediment supply has decreased,
allowing the rivers to begin removing
coarse sediment stored within their
channels. The size, abundance, and stratigraphic
elevation of in-channel gravel bar
deposits increases, channel depth decreases,
and the frequency of overbank flooding increases
downstream along these channels.
This is presumed to be a transient state and
continued transport will lead to degradation
of the bed in downstream sections as
the channel adjusts to the modern supply
of water and sediment. A downstream decrease
in channel size is contrary to the expected
geometry of self-adjusted channels,
but is consistent with the presence of partially
evacuated sediment remaining from
the earlier agricultural period. Reverse
(downstream decreasing) channel morphology
is not often cited in the literature, although
consistent observations are available
from areas with similar land-use
history. Identification of reverse channel
morphology along individual watercourses
may be obscured in multiwatershed compilations
in which other factors produce a
consistent, but scattered downstream trend.
Identification of reverse channel morphology
along individual streams in areas with
similar land-use history would be useful for
identifying channel disequilibrium and anticipating
future channel adjustments.