Headwater Rivers
& Carbon Storage
Ellen Wohl
Geosciences
Colorado State University
Context
Regional Setting
C Pools along Front Range Rivers
Implications
Context
Watersheds are sites for
terrestrial & aquatic CO2 removal thru photosynthesis
transport of living & decomposing organic C in
surface & ground waters
*
storage of organic C for widely varying lengths of time
Amount of C rivers deliver to oceans is a fraction of that
entering rivers from terrestrial ecosystems
atmosphere
1.2
land
2.7
rivers, lakes, wetlands
0.9
ocean
0.6
geosphere
Aufdenkampe et al., 2011
Headwaters are important
significantly contribute to CO2 outgassing via
microbial activity (Battin et al., 2007)
transient storage greatly affects nutrient uptake –
secondary channels, logjams, algal mats, hyporheic,
floodplain (Battin et al., 2008, 2009)
storage also affects sequestration at time scales of
102-103 years
vegetation
soil
landslides
C Pools
C
gradual erosion
vegetation
dead
wood
floodplain
fossil C from bedrock
stream
litter
duff
roots
Basic research questions
Where are carbon and fine sediment stored in a
mountainous headwater river network?
What is the magnitude of storage in different
segments of the river network?
What are the mechanisms facilitating storage?
How might mechanisms and magnitude of storage
change with anthropogenic alteration of rivers and
the greater landscape?
Regional Setting
Colorado Front Range
mountain rivers, with varying valley morphology, as
reflected in process domains based on
elevation (hydroclimatology)
valley geometry (lateral confinement)
biotic drivers (forest age, beavers)
unconfined
confined
Basin-scale heterogeneity (bedrock jointing, glaciation)
102-103 m, 103-105 y
differences in valley geometry
(confined/unconfined)
Reach-scale heterogeneity (logjams, beaver dams)
101-102 m, 101-102 y
differences in channel geometry
(single-thread/multi-thread)
Unit-scale heterogeneity
differences in C retention &
biological uptake
upstream from jam
mountain river sluices?
not quite
Unconfined valleys have the potential for a multi-thread
channel planform driven by biota
channel-spanning logjams (old-growth forest)
beaver dams (beaver)
Channel-spanning logjams & multi-thread channels
old-growth forest
logjams
multi-thread channels
avulsion/
multi-thread
bank erosion,
overbank flow
wood
recruitment
upstream alluviation for length
of > 2X channel width
(persistent)
treefall
ramped piece
logjam
shallow, wide valley
threshold based on gradient &
channel width/valley-bottom width
steep, narrow valley
upstream alluviation for length
of 1-2X channel width
(transient)
Wohl, 2011
Beaver dams & multi-thread channels
disturbance ?
aspen
beaver
high
water
table
multi-thread
channels
Polvi & Wohl, 2012
aspen
&
willow
prior to intensive human manipulation of forests & rivers,
patches of old-growth & beavers more widespread
headwater multi-thread channels more common
greater complexity & retention
Historical changes
loss of old-growth forest
(globally, reduced forest cover by half & nearly eliminated old growth)
currently 6-12 million beaver in North America
historically, more like 50-125 million
evidence of much more extensive beaver activity on east side RMNP
e.g., Upper Beaver Meadows – beaver-induced sedimentation
accounts for 30-50% of post-glacial sediments
C Pools along Front Range Rivers
Objectives
quantify C in different reservoirs in diverse valley types
7 valley types based on
lateral confinement
channel planform (single vs multi-thread)
biota (old-growth or younger forest, beaver)
C Pools
floodplain soils
floodplain coarse wood
instream coarse wood
floodplain fine organic matter
litter
duff
floodplain vegetation
live standing trees
dead standing trees
tree regeneration (understory)
shrubs
herbaceous plants
roots (trees, shrubs, herbaceous)
Methods
100 m valley length
11 transects at 10 m spacing
soil thickness at 10 m increments
all floodplain & instream wood
biomass
trees: dbh, height, % live & dead canopy
along transects
understory trees: count by height class
(3 growth stages) along transects
shrubs, herbaceous: estimation of cover (0.5 x 0.5 m) plot along transects
litter, duff: sample 0.2 x 0.2 m plots along transects
roots: estimate values from literature
Preliminary Results
channel surveys on eastern side of Rocky Mountain NP
> 120 river km in 16 drainages
of total river length surveyed
14% beaver meadows
11% unconfined (3% unconfined multi-thread)
32% partly confined
43% confined
(23% old-growth)
maps by
Nick Sutfin
Where are carbon and fine sediment stored in a mountainous headwater
river network?
Because of greater valley bottom area, greater sediment
thickness, & greater basal area of forest, predominantly in
beaver meadows & unconfined, old-growth valley segments
What is the magnitude of storage in different segments of the
river network?
Unconfined valley segments < 25% of total river length,
but contain 75% of the C in valley bottoms (~ 20% of
total C in watershed)
Megagrams of carbon
1000
3000
2000
unconfined
partly
confined
younger
old-growth
younger
old-growth
newly abandoned beaver meadow
long abandoned beaver meadow
old-growth multi-thread
old-growth single-thread
2000
wood
vegetation
sediment
0
1000
confined
Wohl et al., in review
What are the mechanisms facilitating storage?
Variations in rock erodibility, glacial history & biotic drivers
How might mechanisms and magnitude of storage change with
anthropogenic alteration of rivers and greater landscape?
In the absence of biotic drivers, valley geometry does not
change, but channel-valley bottom interactions change &
C in living and dead biomass changes
water
sediment
nutrients
beaver meadow
plan
side
water
sediment
nutrients
Implications
• leaky rivers as biotic drivers lost
• Front Range rivers not unique in terms of simplification
• alternative stable states for rivers in terms of
logjams
beavers
Wohl & Beckman,
in press
Polvi & Wohl, 2012
• restoration?
How do individual segment Wc/Wv
scale with increasing drainage area?
How does floodplain turnover time vary
among segment types? With increasing
drainage area?
What is role of disturbance
(fire, flood, debris flow, blowdown, insects)?
Timing is everything …
References
Aufdenkampe et al., 2011, Riverine coupling of biogeochemical cycles between
land, oceans, and atmosphere. Frontiers in Ecology 9, 53-60.
Battin et al., 2007, Microbial landscapes: new paths to biofilm research.
Nature Reviews 5, 76-81.
Battin et al., 2008, Biophysical controls on organic carbon fluxes in fluvial
networks. Nature Geoscience 1, 95-100.
Battin et al., 2009, The boundless carbon cycle. Nature Geoscience 2, 598-600.
Polvi & Wohl, 2012, The beaver meadow complex revisited – the role of beavers
in post-glacial floodplain development. Earth Surface Processes & Landforms
37, 332-346.
Wohl, 2011, Threshold-induced complex behavior of wood in mountain streams.
Geology 39, 587-590.
Wohl & Beckman, in press, Leaky rivers: implications of the loss of longitudinal
fluvial disconnectivity in headwater streams. Geomorphology.
Wohl et al. in review. Mechanisms of carbon storage in mountainous headwater
rivers. Nature Communications.