Rivers Gone Wild: Observations of increasing peak flows, greater sediment inputs and channel response in glacial rivers of Western Washington: Signs of what is to come?Presented by:


22 févr. 2014 (il y a 3 années et 1 mois)

62 vue(s)

USGS WaWSC Seminar Series:

Seminar Specifics:

Wednesday, October 6, 2010, noon

Rivers Gone Wild: Observations of increasing peak flows, greater sediment inputs and
channel response in glacial rivers of
Western Washington: Signs of what is to come?

Presented by:

Paul Kennard
, Regional Geomorphologist from

Mount Rainier National Park


The magnitude and rate at which glacial landscapes respond to climate change is a critical question for river
agement. Climate
induced changes to stream flow and sediment supply have direct impacts to how we evaluate
flood hazards and design infrastructure. Glacial recession has exposed large volumes of high elevation,
unconsolidated, and over
steepened sediment w
ithin the headwaters of glacial rivers. Observations of greater rainfall
precipitation correspond to noted increases in atmospheric freezing levels. Rainfall events are in
turn associated with
increased frequencies of debris flows and flooding, that genera
te and carry sediment downstream. Historic peak flow
records of rivers draining Mount Rainier, and other glaciated systems in the Western Washington, show a striking
increase in the occurrence of large floods. Six of the largest recorded storms in the Paci
fic Northwest have occurred
in the last 25 years, with the December 2007 storm ranking as the heaviest 12 hour precipitation ever (Parzybok et
al. 2009). Observations are further supported by initial hydrologic modeling of the Willamette basin, predicting
greater variance in peak flows (increases and decreases in top and bottom 5 percentiles, respectively, Chang 2009).
The most direct evidence of river response to these changes in the alpine landscape and climate is channel
aggradation and burial of ripar
ian forests documented on several of Mount Rainier’s rivers. River sedimentation
suggest that sediment supply from receding glaciers is out
pacing any increase in transport capacity resulting from
larger storms, as reflected in the November 2006 floods tha
t closed Mount Rainier NP for over 6 months.

Wednesday, October 13, 2010, noon

The Role of Riparian Forests in Floodplain Disequilibrium in an Aggrading Braided River:
A Newly Recognized Model in Fluvial Geomorphology

Presented by:

Tim Abbe
, Vice
ent and Technical Director from

ENTRIX, Seattle, WA


The upper White River is a glacially sourced braided river draining Mount Rainier.

The mainstem braided river
channel exhibits remarkable lateral stability set between old
growth forest floodp
lains that are several centuries old.
Many of the floodplains are situated 3 to 6 m below the river channel and pose significant opportunities for channel

The lateral floodplain gradients can be more than twice that of the main channel.

are indeed
multiple cases where river flows have flowed into the floodplains where they rapidly carve new channels. Yet we
find no instances where the river re
directed its course through the forest.

In these situations the forests can suffer
mortality but in all of the historic cases we’ve examined new forest is established where the old forest
had been.

We believe the forests provide sufficient energy dissipation to prevent major avulsions that would re
position the river channel. Evidence f
rom Tahoma Creek on the Southwest flank of Mount Rainier indicates that
accelerated rates of channel aggradation may overwhelm the forest influence and bring up important implications
linked to climate change.

Wednesday, October 26, 2010, noon

Aggradation in Puget Lowlands

Presented by:

Jon Czuba
, Hydrologist

from the

Washington WSC


On Mount Rainier, Washington, the National Park Service has documented widespread

aggradation of as much as
10 m since the early 20th century, of riv
ers draining the glaciated stratovolcano. This rapid sedimentation appears to
be related to glacial retreat and also may be a function of the increased magnitude and timing of peak flows that
mobilize and transport sediment. We are conducting an assessment

of the Puget Lowland rivers that drain Mount
Rainier, 25
100 km downstream from the park boundary, to document the geomorphic response of the downstream
reaches given the widespread aggradation upstream. These downstream reaches provide critical aquatic h
abitat for
spawning and rearing of several species of salmonids, including endangered Chinook salmon and steelhead. Fluvial
sedimentation can have both deleterious and beneficial effects on aquatic habitat depending on sediment particle
size, river slope a
nd width, and river management. To date, our work shows sedimentation of as much as 2 m
between 1984 and 2009 in these lowland rivers. Aggradation rates that were calculated by comparing channel
change at 156 cross sections, ranged between 4.8 and 9.1 cm/y
r in reaches where rivers exit the mountain front and
enter the lowland. Analysis of streamflow
gaging station data from throughout the watersheds draining Mount
Rainier show rapid incision and aggradation, suggesting pulses of coarse
grained bedload may b
e moving down the
mountainous rivers as kinetic waves. Preliminary results, however, seem to indicate that the rivers in the Puget
Lowland have not yet experienced significant widespread sedimentation directly related to glacial retreat. Estimating
the tim
e of arrival of mobilized alluvium is a critical need for resource managers given the potential effects of
sedimentation on river flood
conveyance capacity, fish habitat, and estuarine wetlands.