Suspended Solids and Organic Matter on the St. Croix River
and Implications for Freshwater Mussels
Carl Skarbek, Dan Hornbach, Kelly MacGregor, Mark Hove
Macalester College – St. Paul, MN
-Geology and Biology Departments-
Introduction
Mussel density vs. Total Annual Organic Matter
18
Fig 10 (right). Mussel density at
three sites plotted against the total
organic matter (food supply) at
those sites. More data is needed in
order to predict a relationship
between mussel density and food
quantity.
SSC vs. Water Discharge 2008-2009
0.028
0.026
0.024
0.022
0.020
0.018
0.016
30000
Nevers
HQ
25000
USGS
Franconia
20000
Water
Discharge
15000
0.014
0.012
0.010
10000
0.008
Water Discharge (cfs)
Fig 5 (above). Rating
curve with 95%
confidence interval for
data on the left
0.006
5000
0.004
12
10
8
6
Franconia
Nevers Dam
4
USGS
2
0
5000000
5500000
6000000
6500000
7000000
7500000
8000000
8500000
Organic Matter (Kg)
Total Suspended and Organic Sediment 2008-2009
14000000
12000000
10000000
0.002
0.000
10/07/07
14
16000000
Kg
Sediment transport is one of the most important aspects of a
dynamic river system. It is especially important for the 40
species of freshwater mussels on the St. Croix River, as
they are filter feeders and rely on organic sediment
suspended in the water column to obtain nutrients, while
inorganic matter can clog their filtering apparatus. This is
part of a larger project to create a sediment budget for the
St. Croix.
16
01/15/08
04/24/08
08/02/08
11/10/08
02/18/09
0
09/06/09
05/29/09
8000000
6000000
Date
4000000
2000000
The purpose of this project is to examine the quantity of
suspended sediment and organic matter in the river at four
sites during the annual flow cycle. We are interested in
sources and sinks of suspended material between Nevers
Dam and Franconia, and seek to understand factors
controlling the entrainment and transport of suspended
material. We examined possible correlation between the
amount of organic material and mussel density.
Methods
0.009
0.008
30000
Nevers Dam
HQ
USGS
Franconia
Water Discharge
20000
0.006
0.005
15000
10000
0.003
-Each sample was then pumped through a vacuum filtration
system to filter out sediment particles (Fig 1).
-The filters were then dried for one hour and weighed to
obtain a total suspended solids concentration (TSS) (Fig 2).
0.002
5000
0.001
0.000
10/07/07
01/15/08
04/24/08
08/02/08
11/10/08
02/18/09
05/29/09
HQ SSC
HQ Organic
0
09/06/09
Date
Fig 6 (above). Suspended organic matter at four sites on the St. Croix plotted against water
discharge. Like SSC, water discharge also predicts suspended organic matter. The most
dramatic peaks in organic sediment coincide with the spring floods.
Stillwater and Prescott Suspended Solids vs. Water Discharge 2000-2004
35000
Suspended Solids (mg/L)
Water Discharge
30000
40
25000
35
-The total suspended organic matter was found by taking the
difference of the total suspended solids (weight after drying)
and the suspended sediment concentration (weight after
burning) .
20000
25
15000
20
15
Nevers Dam
10000
10
5000
5
HQ
0
8/29/03
USGS
80.0
Franconia Organic
-Similar annual peaks in suspended organic matter
were observed at Stillwater and Prescott (Met
Council data). The late summer increase in
organics may be connected with algal growth.
45
90.0
Franconia SSC
-Suspended organic matter makes up about 3060% of TSS, with peaks in late summer
(September) and spring (April/May) (Fig. 7).
50
% Suspended Organic Matter 2008-2009
100.0
USGS SSC
USGS Organic
-Water discharge largely controls the TSS and
suspended organic matter (Fig 4&6).
30
-The filters were then put in an oven for one hour at 500°F
to burn off the organic matter and weighed again (SSC).
1
Results Summary
25000
0.007
Nevers SSC
Nevers Organic
Fig 11 (above). The total annual amount of suspended
sediment moving down the St. Croix river is shocking. At
Nevers Dam, there is about 15 million Kg of annual
suspended sediment in transport. That’s about 47,500 adult
male Grizzly Bears worth of sediment in the water column
alone!
Suspended Organic Matter vs. Water Discharge 2008-2009
0.010
0.004
-Surface water samples were collected weekly (biweekly
during winter months) at each site (Fig 3 &8).
0
peak discharge in 2008. The SSC peaks in April coincide with spring snowmelt.
Water Discharge (cfs)
Research Question
Fig 4 (above). Suspended sediment concentrations (SSC) at four sites on the St. Croix
plotted against water discharge. SSC generally follows water discharge at all sites, with
hysteresis (a peak in suspended particles just before a peak in water discharge) at the annual
Fig 12 (left). Data from another study
showing two annual peaks in
suspended solids at locations farther
down the St. Croix River (Met Council,
2005).
0
3/16/04
10/2/04
4/20/05
11/6/05
5/25/06 12/11/06 6/29/07
1/15/08
8/2/08
2/18/09
Date
Franconia
- About 36 metric tons of TSS pass by each site
every day of the year, nearly half of which is
organic and supplies the numerous mussel
populations with sustenance (Fig. 11).
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
10/07/07
01/15/08
04/24/08
08/02/08
11/10/08
02/18/09
05/29/09
09/06/09
Date
Fig 7 (above). The organic fraction of suspended sediment fluctuates dramatically throughout
the year. The average amount of organic sediment is 37%. There are obvious peaks in amount
of organics in the early spring and late summer. Overall, the site at Franconia has a higher
-There is an annual loss of nearly 2500 metric tons
of sediment between Nevers Dam and HQ,
suggesting that sediment is being deposited in the
reservoir above the St. Croix Falls Dam.
-It is interesting to note that Nevers Dam and
USGS sites have nearly identical suspended
organic sediment and mussel density, but more
data and analysis is needed to determine if organic
matter has a role in determining mussel
distribution (Fig. 10).
percentage of organic matter than any other site.
Fig. 1 Water filtration system used to filter
out sediments from river water samples
Future Analysis
-Examination of what makes up the organic
portion of TSS
Fig 3. Sites on the St. Croix
where weekly water samples
were taken. The St. Croix Falls
Dam is also shown.
-More detailed consideration of organic sediment
transport with bedload during spring floods
Fig 8 (above). Karen Jackson taking a water sample
Fig 9 (above). Mussel sampling at
Franconia
Fig. 2 Weighing filters
Acknowledgements: Thanks to Karen Jackson for weighing filters all winter long, the MN
DNR for their support in funding and and collecting data for us on this project, and to
Dan, Kelly, and Mark for their steadfast leadership in sometimes murky waters.
-Exploring additional data to determine the causes
of peaks in suspended organic matter
-Obtaining and analyzing more data on mussel
distributions relating to suspended organic matter