Table Of ContentHistoric, Archive Document
Do assume
not content reflects current
scientific knowledge, policies, or practices.
Reserve
aTC175
.2
.K58
2009
Characterization of "Reference" Suspended-
Sediment Transport Rates for Level III
Ecoregions of Minnesota
Lauren Klimetz and Andrew Simon
National Sedimentation Laboratory Technical Report 63
USBA
April 2009
United States
Department of
Agriculture
National
Agricultural
Library
AdvancingAccess to
Global Information for
Agriculture
Characterization of "Reference" Suspended-
Sediment Transport Rates for Level III
Ecoregions of Minnesota
^^^^
^ 7
Prepared by
U.S. Department ofAgriculture - Agricultural Research Service
National Sedimentation Laboratory
WatershedPhysical Processes Research Unit
Oxford, Mississippi
April 2009
CHARACTERIZATION OF "REFERENCE" SUSPENDED-
SEDIMENT TRANSPORT RATES FOR
LEVEL ECOREGIONS OF MINNESOTA
III
ARS Designated Representative and Project Manager:
Carlos V. Alonso
Technical Direction
Andrew Simon
Report Preparation and Data Analysis
Lauren Klimetz and Andrew Simon
Mapping and GIS
Danny Klimetz
Field Operations and Database Management:
Lauren Klimetz
Field Data Collection and Data Processing:
Brian Bell, Bruce Butler, Danny Klimetz, Lauren Klimetz and Ibrahim Tabanca.
"Reference" Suspended-SedimentTransportRates: Minnesota iv
EXECUTIVE SUMMARY
As a result of 'natural' and human-induced changes to precipitation, runoff production
and land use water-quality issues due to excessive erosion and transport of suspended-
sediment are a primary concern throughout the surface waters of Minnesota. Within
Minnesota there are a number of Level III Ecoregions that are the subject of this
investigation and report: Ecoregion 46, Northern Glaciated Plains; Ecoregion 47, Western
Com Belt Plains; Ecoregion 48, Lake Agassiz Plain; Ecoregion 50, Northern Lakes and
Forest, and Ecoregion 51, Northern Central Hardwood Forests. Rapid Geomorphic
Assessments carried out at current and historical USGS gaging stations found that more
than half of sites visited in Ecoregions 46, 50 and 51 were considered stable, while the
majority of sites in Ecoregion 47 and 48 were found to be unstable. Calculated
suspended-sediment transport yields (load per unit area) at the 'effective discharge' (the
flow that occurs, on average, once every 1.5 years or Qi 5) and the mean annual yield
were sorted into stable and unstable groups based on channel stability information.
"Reference" suspended-sediment transport values were thus obtained from analysis ofthe
distribution of suspended-sediment yields for streams determined to be geomorphically
stable at the time of suspended-sediment sampling. At the Qi 5 discharge, "reference"
yields range over two orders of magnitude; from 0.0039 T/d/km^ for Ecoregion 46, to
0.48 T/d/km' for Ecoreeion 47. Mean-annual "reference" suspended-sediment yield
values also varied by two orders ofmagnitude, from 0.351 T/y/km"" to 20.3 T/y/km~ also
for Ecoregions 46 and 47, respectively. Statistical analysis of the differences between
stable and unstable sites within each Level III Ecoregion were significant at the 0.05
probability level in all cases except Ecoregion 51 Qi 5 yield values. However, stable and
unstable values were statistically significant with the removal oftwo major outliers. This
provides support for the overall approach ofdetermining "reference" or 'target' rates for
suspended-sediment transport and the development ofTMDLs in Minnesota.
Where suitable data were available, other distinctions were made within each Level III
Ecoregion. The majority of sites in all Ecoregions were dominated by a sand bed
material (median diameter between 0.063 - 2 mm). There were, however, sufficient
coarse-dominated stable channels (median particle diameter greater than 2 mm) within
Ecoregions 47, 50 and 51 with which to calculate embeddedness for these Ecoregions.
Embeddedness was calculated as the percentage of fine material present in a coarse
dominated bed material. "Reference" values ofembeddedness lie between an acceptable
range of 5 and 10 %. Suspended-sediment yield data were also sorted by drainage-area
size-classes and individual watershed in some cases, however insufficient data existed
within each size-class to establish statistically significant trends, and the possibility of
developing a "reference" transport rating equations for each Level III Ecoregion
investigated.
All sediment-transport data were re-characterized in terms ofthe frequency and duration
ofsuspended-sediment concentrations. Data in this format are potentially more useful to
biologists and aquatic ecologists seeking functional links between sediment transport and
biological metrics. In addition sediment 'dosage' was calculated as the product of
concentration and duration and was also differentiated by stability class. 'Dosage
"Reference" Suspended-SedimentTransport Rates: Minnesota V
impact' then represents the difference in sediment dosage for stable and unstable
conditions at a given concentration exceedance. Median suspended-sediment
concentrations (in milligrams per liter) are lower in stable or ''reference" channels at a
given percent exceedance (or frequency) than in unstable channels in Ecoregions 46 and
50. Therefore, unstable channels consistently transport higher concentrations of
suspended-sediment than stable channels in the both the Northern Glaciated Plains and
the Northern Lakes and Forests. In Ecoregions 47, 48 and 51 this is true during moderate
%
and high flows, those exceeded only 20 ofthe time. This same pattern is evident when
considering the frequency-magnitude and duration ofconcentration events between stable
and unstable channels within each ecoregion. Sediment-dosage data show distinct
differences between stable and unstable sites across the range of concentrations for the
most part in all Ecoregions in Minnesota. Ecoregion 47 has the highest dosage values, in
terms of both stable and unstable channels, while Ecoregion 50 has the lowest, and the
clearest separation between stable and unstable channels. "Reference" suspended-
sediment dosage peaks during frequently exceeded concentrations; those greater than 99
%
of the time, therefore low concentration events. The only exception to this is
Ecoregion 46, where data are skewed by ephemeral streams. This implies that low
frequency, high concentration events may have a greater affect on sediment-sensitive
organisms.
