East Nishnabotna River IA 05-NSH-1414
mouth (S2 T67N R42W Fremont Co.) to confluence with Fisher Cr. in S27 T69N R40W Fremont Co.
- Cycle
- 2026
- Current Phase
- Public Comment
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trend
- Stable
- Created
- 10/27/2025 3:10:44 PM
- Updated
- 1/15/2026 8:02:15 AM
The Class A1 use remains assessed as "not supported" due to levels of indicator bacteria (E. coli) that exceeded state water quality criteria. The Class BWW1 use was assessed as "not supported" based on a fish kill investigation and "WINOFI" based on biological sampling data. The Class HH (fish consumption) use was assessed as "fully supported" based on fish tissue sampling data.
Data Sources: Data Source Data Source ID Data Type Data Age Site ID Site Name Site Description Iowa DNR 6 WQ CY 2004-2006 10360002 East Nishnabotna
River Downstream of Shenandoah DS1 Iowa DNR 6 BIO See Below 1298 East Nishnabotna
River Riverton (REMAP
#400) Iowa DNR 6 FT See Below 1083 East Nishnabotna
River W of Riverton Site ID Data Source ID # Samples / # Years 2004-2006 Geometric Mean Annual Geometric Mean Violation # Violations % Violations Significantly >10% Violations Assessment Type Support Level 10360002 6 24 / 3 272 Yes 15 62% Yes Evaluated Not Benthic
Macroinvertebrates: Site ID Date Drainage Area (mi2) Ecoregion Major Drainage BM Sample Gear BMIBI score BMIBI Class 47e BMIBI BIT BMIBI Pass/Fail 1298 9/24/24 1110.0 47e MO Floating Art Sub 22 Poor 54 Fail 1298 8/19/24 1110.0 47e MO Floating Art Sub 36 Fair 54 Fail 1298 8/16/21 1110.0 47e MO Floating Art Sub 20 Poor 54 Fail Site ID Date Drainage Area (mi2) Ecoregion Major Drainage Riffle Stream? FIBI score FIBI Class 47m FIBI BIT FIBI Pass/Fail 1298 9/25/24 1110.0 47e MO NA 43 Fair 31 Pass 1298 8/16/21 1110.0 47e MO NA 6 Poor 31 Fail Community Assessment Type Assessment Result Support Level IR Category Reason Benthic
Macroinvertebrates Evaluated Fail (0/3) WINOFI Fish Evaluated Fail (1/2) WINOFI Assessment was
evaluated because the drainage area Overall Evaluated Fail WINOFI 3bu of the sample site
was larger than the calibrated range. Class BWW1 – Fish kill Assessment: Following DNR IR fish kill methodology, the Class BWW1 aquatic life use
assessment based on the March 2024 fish kill should have resulted in a Category
3b potential impairment because a single fish kill occurred in this segment
during the IR data assessment period (2020-2024). However, given the magnitude
of the fish kill (~50 miles affected and ~750,000 fish killed), DNR IR staff
felt it was appropriate to place this segment in Category 4d. The fish kill was
caused by a “pollutant” or was suspected to be caused by a “pollutant”;
therefore, the Class BWW1 use was considered impaired. Since restitution for
the fish killed was received and/or the responsible party paid a fine, the
Class BWW1 use was placed in Category 4d (impaired but TMDL not required) and
not in Category 5b (impaired and TMDL required). DNR feels that (1) TMDLs
should not be required for a fish kill caused by a one-time illegal or
unauthorized release of a pollutant where enforcement action was taken and (2)
enforcement action is more appropriate and effective for addressing a
spill-related impairment than the TMDL process. In September of 2024, DNR Water Quality and Fisheries Bureaus conducted follow-up
biological sampling on this segment. The fish, benthic macroinvertebrate, and
freshwater mussel communities were sampled along with the collection of a grab
water quality sample and a physical habitat assessment was conducted. Results
from that sampling indicated that the biological communities had recovered to
pre-fish kill levels and were comparable to similarly sized sites in the
ecoregion. Details
about the 2024 fish kill can be found here: https://programs.iowadnr.gov/fishkill/events/1045 Details about the DNR
Contaminated Sites Section’s oversight can be found here: https://programs.iowadnr.gov/contaminatedsites/Site/Detail/2818 Details about the U.S.
Fish & Wildlife Service’s oversight can be found here: https://www.fws.gov/project/nishnabotna-fertilizer-spill Site ID Biopart Year Species # in sample / # of samples Avg. Length (cm) (Avg.) Hg (ppm) Chlordane (ppm) Sum PCBs (ppm) 1083 fillet 2024 common carp 3 49.4 0.1 <0.0501 <0.0246 1083 tissue plug 2021 common carp 10 60.51 0.151 1083 tissue plug 2020 channel catfish 10 48.66 0.12 Support Level Full
Class A1 - Indicator Bacteria: 2004-2006
Class BWW1 - Biological Sampling:
Fish:
Overall:
Fish and benthic macroinvertebrate biological sampling data from this segment can be found here: https://programs.iowadnr.gov/bionet/sites/1298
Class HH - Fish Tissue Sampling: