Analysis of Lake Pend Oreille waters has indicated 

 relatively low nutrient concentrations in the open water 

 areas but significantly greater evidence of eutrophication 

 in developed and confined bays. Relatively little informa- 

 tion is available regarding nutrient sources in Lake Pend 

 Oreille. The Clark Fork, which contributes 90 percent or 

 more of the annual inflow of water to Lake Pend Oreille, is 

 recognized as an important source of nutrients. Less is 

 known about the contribution of nutrients from other 

 tributaries and from near-shore developed zones. 



Nutrient Concentrations and Loading 



Of the many nutrients required by algae and other 

 aquatic plants, nitrogen and phosphorus are the two elements 

 usually in the shortest supply in natural waters. This means 

 that the growth of algae is often controlled by the con- 

 centration of nitrogen or phosphorus, or both, in the water 

 column. The EPA (1986c) has established criteria values for 

 total inorganic nitrogen and total phosphorus that should not 

 be exceeded in order to prevent excessive developments of 

 attached algae in rivers and to prevent eutrophication in 

 lakes that are fed by rivers. These values are 1,000 ug/1 

 for nitrogen and 50 ug/1 for phosphorus. The criteria may 

 not apply equally well in all situations, and they do not 

 account for other limitations to algal growth. 



WQB data demonstrate that the major sources of nutrients 

 in the Clark Fork Basin are municipal and industrial 

 wastewater discharges. During low-flow years, there is less 

 river water available to dilute the wastewater. This is 

 especially problematic for municipal dischargers, whose 

 discharge rates are relatively constant from year to year. 

 It is less important for some industrial facilities, such as 

 the Stone Container Corporation kraft mill, because their 

 allowable discharges are largely limited by river flow. 



The following summary of FY 85-87 WQB data on river 

 nutrient concentrations and loads may very well represent a 

 near worst-case scenario because of the low streamflow 

 conditions that prevailed during the monitoring period. The 

 generally higher nutrient loading in 1986 probably reflects a 

 greater contribution from nonpoint sources. 



Silver Bow Creek 



Silver Bow Creek from Butte to the Warm Springs 

 treatment ponds suffered from serious nutrient pollution 

 problems on a year-round basis during FY 86-87. Measured 

 concentrations of total phosphorus (Figure 3-27) and total 



3-86 



