Effects of the 1975 Mike Horse Mine Tailings Dam Failure 



on Water Quality and Aquatic Resources of the 



Upper Blaclcfoot River, Montana' 



INTRODUCTION 



On June 19, 1975, the Mike Horse Tailings Dam, located on Beartrap Creek and controlled by 

 the Anaconda Company, in the headwaters of the Blackfoot River near Lincohi, failed due to 

 heavy precipitation and runoff from winter snowpack. Failure of a bypass structure and canal that 

 normally diverted Beartrap Creek flows around the dam caused runoff to fill the pond, overtop 

 the dam and wash out the east abutment. Runoff water carried heavy loads of gray-colored, 

 pyritic tailings out of the pond area and into the Blackfoot River. Temporary repairs made by the 

 company allowed most of the runoff water to be diverted around the tailings pond, reducing the 

 amount of tailings material entering the river below. However, seepage water from within the 

 pond site continued to carry smaller amounts of material into the river for approximately three 

 weeks. The gray-colored water was evident 15 miles downstream before it became obscured by 

 high, muddy waters of Landers Fork, a major tributary stream (See photos at end of report). 



This report summarizes water quality and biological changes which occurred following dam 

 failure. 



Water quality, streambed sediment samples, bottom fauna and fisheries data were collected 

 during the problem period. These data were compared with similar data collected during a 

 baselme study from 1970-1973 in anticipation of a proposed open pit mining venture by the 

 Anaconda Company in the Mike Horse Mine area (Spence 1975). 



A description of the upper Blackfoot River drainage is found in Spence (1975) The Mike Horse 

 Mill was constructed on Mike Horse Creek in 1919 to process lead and silver. Mining and 

 millmg occurred intermittently through the period until about 1954 when the venture was 

 abandoned. During the time the mine was active, a tailings pond was gradually built on Beartrap 

 Creek from mill wastes (See photos). When the dam failed, approximately 100,000 tons of 

 pyntic tailings washed into Beartrap Creek and the upper Blackfoot River (Laird 1975) Metallic 

 constituents of the tails included Cadmium (Cd), Copper (Cu), Iron (Fe), Nickel (Ni), Lead (Pb) 

 Zmc (Zn), Gold (Au), and Silver (Ag). Fe, Pb and Zn were present in the highest concentrations' 



.h,„o,c H ^ "'^ °^^" ""'°" *^ '"^"" "'' *' ^°"^^ ''''•' ^"^ <^' Department in 1976 but never completed due to personnel 



mZ. in mmilln't"" '" TIV" 1>' "'f "•"" "^'"'"^ '''^'^" '" *' "•""' ^'''''°°' '"^" -" P™""« °f °"'" progZs Renewed 

 TcIh f . Ta' ^''"'^°°' '^"'" *'"' ^'"'"^'"^ "^''""'^ '" ^"^- W'l<"'f= & P^^ O^WP) for historic information and FWP 



t reoort rTJ" H "" ""k < 'V" i'"'' "^^ '"'' "" °""^ ^''^'="' *" '^"^^'^ '^^ ^P-^^ ^^ F'^h, Wildlife & pX)^,s is 

 the report referenced in paragraph 5 of the "Repon Supplement" in Spence (1975) 



