“He not busy being born is busy dying.”
- Bob Dylan, It’s Alright, Ma (I’m Only Bleeding)
Clifford Ochs
University of Mississippi
21 June 2016
LMRCC Annual Meeting
June 21-22, 2016
Ducks Unlimited Headquarters, Memphis
There are known knowns. These are things we
know that we know.
There are known unknowns. That is to say, there
are things that we know we don't know.
But there are also unknown unknowns. There
are things we don't know we don't know.
- D. Rumsfeld (2002)
There are known knowns.
P. Hartfield 2014
There are known knowns.
P. Hartfield 2014
H. Fisk
There are known knowns.
Baker et al. (1991)
LMR floodplain backwaters provide critical
ecosystem functions and services
• Habitat complexity  biodiversity
• High biological productivity
– river subsidies?
• Nutrient sequestration
– importance to GOM?
NO3-N (mg/L)
2.0
1.6
1.2
0.8
0.4
• Floodwater storage
AND they are more critical than ever but endangered!
0.0
0
30
60
90
Days post-connection with river
6
LMR-backwater structure and
functional relationships respond
to variation in hydrological
connection.
21 June 2008 (HG = 10m)
14
- 10 year averages from 1900
data from Rivergages
Helena gage (m)
12
10
8
6
6 September 2008 (HG = 1m)
4
2
0
D
J
F
M
A
M
J
J
A
S
O
N
D
J
27 September 2008 (HG = 8m)
At high stage height and connection, the river
delivers sediments and nutrients to BWs.
In the BWs, brown turns to green.
Nutrients
Production
Production
Modoc
In backwaters,
brown  green
Quapaw
chute
Mellwood
400
350
250
200
14
12
10
8
150
6
100
4
50
2
0
0
Stage height (m)
Chl a (mg/m3)
300
River
Quapaw
Modoc
Mellwood
9
Connected river oxbows are a nursery for
high biological production of all trophic levels
O. Pongruktham
Effects of Connection on LMR Backwater Lakes
Shorter HRT
High Turbidity
High Nutrients
Low algal biomass & production
Diatoms dominant
High connection with
river
MORE
RESET AT HIGH
RIVER STAGE
Nutrient depletion
N-limitation of GPP
River Stage
Prolonged
isolation
LESS
Low connection with
river
Pongruktham and Ochs 2015
Longer HRT
Low Turbidity
High Nutrients
High algal biomass & production
Crytomonad algae dominant
What is the fate of lake production?
Nutrients
Production
Production
Biomass
“Cross-Feeding” model: Role of
BWs in river floodplain system
MRB  LMR:
soils
N&P
Cross-feeding
Backwater
production
HOWEVER – Oxbow lakes can and do fill!
Sedimentation disconnection lake infilling
Pauline Dieras (2013)
14
Smoky Hill River
(Kansas and Colorado)
White River
(Indiana)
Pauline Dieras (2013)
Factors affecting lake infilling process
• Land use in watershed
– High soil erosion in river valley  sediment capture in lakes
• Lake shape
– Oxbows are long and narrow = High [shoreline/lake SA]
• Lake productivity
– organic matter sedimentation  summer hypolimnetic anoxia
• Tree encroachment
16
Is there evidence for loss of
lakes within LMR levees?
Horseshoe Lake
Mellwood Lake
Desoto Lake
Old River Lake
Evidence for loss of lakes within LMR levees?
LMR cutoffs
Hardin
Worthington
Glasscock
Cut-off
Date
1942
AVG Rate of
change in SA (%)
-0.6
1933
1933
-5.5
-1.1
Assessment period: 1948-1984
(Shields 1989)
Change in mean depth in comparison bendway sites on TTW = 0.6 ft/yr (SD = 1.8)
(n = 14; period of observation = 3-10 years
Evidence for loss of lakes within LMR levees?
Hudson et al. 2008
Lake infilling rates depend on location
Embanked lakes = 67.7% filled (avg, n=16)
Not embanked lakes = 37% filled (n=8)
Hudson et al. 2008
Evidence for loss of lakes within LMR levees?
Desoto August 2014
Mellwood Nov 2007
Mud flats at downstream oxbow lake entrances
In other words….
Anthropogenic alterations have virtually
halted the formation of new oxbow lakes in
the [Yazoo] basin, and accelerated the
natural aging process, as increased soil
erosion and sediment deposition threaten
to fill the lakes at high rates.
- Goetz et al. (2015). Environ. Biol. Fish. 98
What is the long-term prognosis if embanked
floodplain lakes no longer form but still fill?
“He not busy
being born is
busy dying.”
There are known unknowns.
• At current sedimentation rates, how long can remaining
BW lakes stay connected to the river channel?
• Do rates or pattern of lake infilling vary in time, or river
reach?
• What are the effects of occasional extreme events, large
floods or droughts, on sedimentation processes?
• What would be the river ecosystem consequences of
further loss of connectivity to these lakes?
 Should we put the remaining LMR floodplain lakes on life
support? How?
Should we put remaining LMR floodplain
lakes on life-support? How?
• Re-meandering?
• To weir or not to weir?
• Dredging?
To-Dos for research and management
1. Adopt a long-term view (>20 yrs) of system
geomorphic and environmental change.
2. Assess and Predict
– rates and patterns of lake terrestrialization
processes
– river system structure-function relationships and
services (present, anticipated)
3. Evaluate management strategies to maintain
LMR-floodplain connectivity while reducing lake
infilling.
Summary
1. In a meandering river system, oxbow lakes are regularly born,
and regularly die. Birthing is by river meandering, death is by
sediment infilling. Older lakes are replaced by a younger
generation of lakes. This is a natural process.
2. In a levee constrained river system, natural oxbow lake
formation ceases - although lakes can originate as cut-offs while sedimentation and lake filling can continue.
Lake Birth ≈ Death
Lake Birth < Death
Summary
3. To conserve functional attributes of large lowland river
ecosystems, and their biodiversity, connected lake habitats are
essential.
4. River system management must determine the potential for
oxbow lake infilling, and for breakage in hydrological
connectivity with the river.
5. Manage for connectivity over the long-term.
There are unknown unknowns.
Modern flood management of large rivers is
fundamentally an experimental science, carried out
by large government agencies in trial and error
operations that initiate extensive geomorphic
change with long lasting environmental
consequences.
- Hudson et al. 2008
Acknowledgements
•
•
•
•
Angie Rodgers – LMRCC
US ACE ERDC Ecohydraulics project
F. Doug Shields
Pauline Dieras
https://www.google.com/search?q=Pauline+Dieras&ie=utf-8&oe=utf8#q=Pauline+Dieras+dissertation+oxbows