Uploaded by Josef A Kuklinski

Siuslaw River Basin

Siuslaw River Basin
Josef Kuklinski, Jon Michael | Geology 202 | March 1, 2019
The Formation of the Triangle and Little Lakes
Although there are 1400 named lakes
within Oregon, there are exceedingly few in
the Coast Range. In fact, the glacial and
volcanic processes which are responsible for
most of the lakes within the Cascade Range
are not present in the Coast Range.1 So, what
created these anomalous lakes? The answer
lies somewhere between 2.6 million years ago
and 11,000 years ago during the Pleistocene
The Pleistocene Epoch was
categorized by global cooling and massive
glaciation, including Earth’s most recent
period of repeated glaciations.2 Though the
area in question did not have any glaciers
during this time, it did have a high level of
precipitation due to local climate as well as the
rain-shadow effect. By studying the fossilized
pollen grains in core samples taken from Little
Lake, we get a snapshot of the plants that
were growing in the area during that time and
thus an accurate idea of what the climate was.
From about 42,000 years ago to 25,000 years
ago, the local region was colder and wetter
than present day conditions.3 For comparison,
average annual precipitation for the Oregon
Coast ranges from 60-90 inches near the coast
and all the way up to 200 inches high in the
coast range.4
The Coastal Range during the
Pleistocene was cold and quite moist, but this
cannot be the sole cause as we would expect
to see a great number of lakes in the Coast
Range which is clearly not the case.
Something specific in this region must have
stopped a large amount of water from
draining out of the area for a time. Since
glacial and lava dams are not possible causes
in this case, perhaps a local geologic feature or
event is to blame.
The Coast Range is continuing its
uplift due to stress from the Cascadia
Subduction Zone. The strata are almost
entirely made up of layered marine sandstone
of the Tyee Formation, with volcanic
formations poking through at the top,
forming the ridges of the highest peaks in the
Range.5 Sandstone is easily weathered and
eroded away by water (and gravity) due to its
permeable and porous nature. Thus, the
mystery unravels.
An actively uplifting mountain range
composed of delicate marine sandstone
interspersed with rivers and streams
throughout tends to form narrow river valleys.
If you then add heavy rains (of the
Pleistocene) into the mix, landslides and other
mass-movements become very real
possibilities and this is the picture it paints:
During a particularly fierce downpour at least
42,000 years ago, the bedrock failed on a
northwest-aspect slope near the southern end
of the lake causing a landslide to move down
into the valley and dam Lake Creek, a major
tributary of the Siuslaw River. The lake that
formed would have filled in an area much
greater than today, but lakes are temporary on
geologic timescales and the majority of the
basin has been filled in with sediment. This
sediment deposition also raised the water level
of Triangle Lake above the elevation of Little
Lake, causing a small stream to burst through
the southwest bank and form Little Lake
The Geomorphology of the Siuslaw River Channel
The Siuslaw River is a 110-mile-long
river that morphs from a narrow bedrock and
alluvial channel in the mountains southwest of
Eugene, to a wide, alluvial floodplain before
emptying into the Pacific Ocean near
Florence.6 In this study, we ventured to
investigate and explain what could have
caused this, with a running hypothesis of it
being a combination of discharge and
sediment transport. To understand why this
evolution occurs, though, we must first
discuss discharge and the mechanics of
Water can weather and erode a rock
both mechanically and chemically. One of the
many ways in which water can mechanically
erode bedrock is through sediment transport.7
The kinetic energy of the water combined
with the abrasive qualities of the rocks can
devastate unsuspecting bedrock. Sediment can
be transported by being suspended in solution
as the suspended load, ‘bouncing, sliding, or
rolling’ along the channel bed as bed load, or
dissolved in solution.8 The sediment can be
thought of as an exfoliant, if you will.
A stream’s ability to move a certain
amount of sediment (capacity) and its ability to
move heavier sediment (competency) are directly
related to its velocity and discharge. Though
only velocity is pictured (fig. 1)9, a graph of
Figure 1 Stream Velocity vs. Sediment Diameter
discharge vs. sediment diameter would look
A stream’s discharge is the volume of
water that passes through a given crosssectional area. In other words, discharge is
how much water is flowing downstream at
any given point. This, along with many other
properties, change over the course of a
stream. For example, discharge increases the
further downstream you go due to an increase
in volume from tributaries. The Bradshaw
Model10 illustrates how these different
properties increase or decrease as the water
continues its flow from upstream to midstream.
Now, back to the question at hand:
why does the Siuslaw River transition from a
narrow bedrock and alluvial channel near its
headwaters to a wide, alluvial floodplain near
its mouth? The volume of water flowing
through a stream is at its lowest near the
headwaters as no tributaries have added to it
yet. Since the volume of water is low, we can
deduce that both discharge and velocity are
lower as well. In practical terms, this translates
to a stream that is not yet flowing strongly
enough to remove much material and gouge
out a wide channel, instead carving a narrow
channel to fit the needs of the discharge level.
As we continue down the river, the
gradient, or slope, begins to decrease all the
way down to nearly-flat at the ultimate base
level. At this point, the water has gained in
both velocity and volume. With a higher
capacity and competency, the stream is now
flowing with enough energy to erode the
channel bed and walls, deepening and/or
widening the stream until a dynamic
equilibrium is achieved between discharge and
channel shape. When the stream is wider, a
smaller proportion of the river is experiencing
friction with the channel bed and walls
resulting in a net positive velocity. Eventually
the gradient will decrease to such an extent
that the river is forced to slow down.11
It’s important to note here that the
shape of a river is defined during ‘Extreme
High Water’ events when kinetic energy is at
its maximum and that mass simply must be
preserved. The discharge is still at its highest,
or nearly highest point, and that water must
go somewhere. If the discharge is high
enough, the river will overflow its banks and
flood the surrounding flatlands. Since the
discharge is so high at this point, massive
amount of sediment is eroded and transported
downstream or out into the floodplains.12 As
the sediment-laden water spreads out across
the flatlands, it slows down, decreasing its
capacity and competency forcing its
suspended load and bed load to be deposited.
This process eventually results in a wide, fanlike alluvial floodplain as the discharge shapes the
river to fit its needs.
Building a Beach: Surface Process Survey at Heceta Head
Aerial Photograph of Heceta Bay and lighthouse
Standing on the extreme precipice of
the Oregon Coast overlooking a scene of
chaos: white, frothing waves come barreling
towards the coast, breaking themselves with
terrible vengeance upon the sea stacks. This is
a situation one can encounter at numerous
locations along the Oregon Coast, but it raises
a troublesome question: how can the
headland, sea stacks, and beach holdfast under
such conditions? Shouldn’t they be
disappearing? Well, the answer is a simple
one: They are disappearing, quite violently in
In the picture above, you can see that
the waves approach the shore in a straight
line, yet perfectly conforms to the shape of
the land as they collide with the land. This is
achieved through a process known as wave
refraction, and it either disperses the wave’s
energy (the beach) or concentrates it (the
cliffs).13 The wave energy is incredibly
destructive to the cliff and creates numerous
erosional features such as sea caves, sea
stacks, and sea arches. The sea stacks shown
here are remnants of a headland that is, well,
no longer with us. The mere fact that these
headlands have formed tells us a lot about the
local geology and surface processes.
A headland is “an elevated area of
hard rock that projects out into an ocean or
other large body of water.”14 These are
formed in strata that are alternatively soft and
hard so as to cause a differential erosion
pattern.15 As the soft stone is eroded out from
underneath a harder layer, they eventually
form sea arches which erode and collapse
leaving isolated sea stacks not unlike the ones
pictured above. My observations on the beach
confirm this (see pictures below).
it is estimated that streams and rivers move
about 1.65 billion tons of sediment from land
to the oceans each year.17
In conclusion, the answer is yes, many
of the features along the coast including at
Heceta Head are disappearing. There is no
reason to fear, though! Due to the continuous
uplift this region experiences, more cliffs and
strata are being exposed to replace that which
was lost!18
The figure above shows a large layer
of soft sedimentary stone overlying the much
harder layers of basalt. Here, it is easy to see
the differential weathering.
Another massively important factor that goes
into shaping a beach: the sand! More
specifically, the input and output of it. As
waves wash up on the beach, they deposit
their suspended load, however the backwash
created as the wave retreats removes
sediment. This effect is amplified during
stormy conditions when the backwash is of
much greater magnitude than the swash.16
There is also the longshore current
which moves sand along our coast from north
to south due to the Coriolis effect. However,
like the waves, the input of the longshore
current is lessened because it removes sand
from offshore as well. The largest input of
sediment onto the beach would have to be the
Siuslaw dumping the rest of its suspended
load along its banks and in the estuary. In fact,
Hale, Jamie, and Jamie Hale. “25 Beautiful 'Real'
Lakes in Oregon.” OregonLive.com, OregonLive.com, 16
Aug. 2018, expo.oregonlive.com/life-and-culture/erry2018/08/b663d4d3391150/25-beautiful-real-lakes-inore.html.
2 Johnson, W. Hilton. “Pleistocene
Epoch.” Encyclopædia Britannica, Encyclopædia
Britannica, Inc., 30 July 2018,
3 Worona, Marc A., and Cathy Whitlock. “Late
Quaternary Vegetation and Climate History near Little
Lake, Central Coast Range, Oregon.” Geology,
GeoScienceWorld, 1 July 1995,
4 “Siuslaw Watershed.” Oregon Explorer,
5 Armantrout, N.B. “Watershed Analysis and
Restoration in the Siuslaw River, Oregon, USA.” The
Canadian Journal of Chemical Engineering, Wiley-Blackwell,
16 Apr. 2008,
6 Sullivan, William L. Atlas of Oregon Wilderness. Navillus
Press, 2014.
7 “Rivers and Streams - Water and Sediment in
Motion.” Nature News, Nature Publishing Group,
8 “Stream and River.” Science Clarified,
Data, US Climate. “Temperature - Precipitation Sunshine - Snowfall.” Map of Eugene - Oregon - Longitude,
Altitude - Sunset,
10 Borneman, Elizabeth. “How Rivers Change the
Landscape.” GeoLounge: All Things Geography,
GeoLounge: All Things Geography, 24 Apr. 2014,
11 Board of County Commissioners - Lane County,
Britannica, The Editors of Encyclopaedia.
“Diffraction.” Encyclopædia Britannica, Encyclopædia
Britannica, Inc., 14 Apr. 2014,
14.“Landforms of Erosion.” A Level Geography,
15 “Headland Landforms.” World Landforms,
16 “Coast and Shore.” Science Clarified,
17 “Stream and River.” Science Clarified,