10 years of nutrient reductions on Montana’s Clark Fork River

Background/Question/Methods

In the 1980’s much of western Montana’s Clark Fork River was identified as impaired by nuisance benthic algae as a result of excess nutrients. Basin stakeholders developed a voluntary nutrient reduction plan (VNRP) which the EPA substituted for a mandatory TMDL. Nutrient and algae studies, mass transport models and best professional judgment were used to set targets for acceptable algae levels and to estimate nutrient levels and loads expected to meet those targets. The 10 year plan was adopted in 1998 and ended in 2008. Monthly monitoring of nutrient levels and of summer attached algae levels used standard methods and an EPA approved QA/QC plan.

During the VNRP, the major point source dischargers to the river reduced Total Nitrogen loads by 18% and Total Phosphorus loads by 66%. While new homes were being built with septic systems, the river's largest city Missoula connected many more old homes to the sewer, resulting in a net reduction of over 3,000 septic systems. The Tristate Water Quality Council funded nonpoint nutrient reduction programs on the river’s tributaries.

Results/Conclusions

Over the 10 years of the VNRP, in-river levels of Total Nitrogen, Total Phosphorus and Soluble Reactive Phosphorus decreased significantly, but Dissolved Inorganic Nitrogen did not decline.

Achievement of nutrient targets in the last three full years of the program reached over 90% of samples for total nitrogen and over 70% for phosphorus in the middle river.  Compliance with targets in the upper river was lower, partly because total phosphorus targets are stricter there. Benthic algae targets were met in only 30% of the samples in the upper river and 70% of the samples in the middle river.

Trend analysis of algae levels showed improving or static conditions in the middle river, but static or increasing levels in the upper river. The upper river is dominated by the difficult to control Cladophora glomerata.

 Flows play an important role in attached algae levels in streams. Stream discharge was  substantially higher the first few years of the VNRP. The lower flows later in the program likely provided less scour of algae, and could have resulted in nitrogen-rich groundwater being a higher percentage of stream flow.

 Monitoring has continued since the VNRP ended, and the more recent data are under analysis now and will be included in the presentation.