The floodplain of the Clark Fork River between Warm Springs and Garrison, Montana, is contaminated with fine mine tailings deposited during several large floods approximately 100 years ago.  If left alone and given enough time, the channel of the Clark Fork River would naturally rework the contaminated sediment and eventually remove most of it from the valley.  Based on the thickness of the contaminated sediment deposits, the tailings raised the level of the floodplain on the order of 10 to 30 cm within the meander belt of the river, changing the bankfull capacity of the channel.  A large tailings basin constructed around 1918 and enlarged periodically until about 1995 controls flow from approximately 60 percent of the contributory watershed area at the upstream end of the study reach.  Both the tailings themselves and the tailings basin have affected the hydraulic and hydrologic characteristics of the system.  Since the tailings basin currently captures most incoming contaminated sediment, the floodplain downstream of the basin represents one of the most significant sources of contamination for the system.  The time required for the natural removal of the contaminated sediment from the floodplain is controlled primarily by the width of the floodplain and the rate at which the channel migrates laterally.  This project involves the development of a model for identifying the time scale for natural attenuation of the contaminated sediment wave as it travels through the floodplain and to assess in a general sense the long term evolution of a typical channel cross section.The model assumes a decoupling between the volume of sediment eroded from the floodplain due to bank migration and the volume deposited in point bars or on the floodplain during flood events, allowing channel geometry and bankfull capacity to evolve over time in response to historic changes in floodplain elevation.  Preliminary results show that without human intervention, tailings could remain on the floodplain for thousands of years.  It also indicates that contamination of river floodplains with pollutants that are adsorbed to fine sediment occurs primarily because of an overloading of fine sediment at levels much higher than the natural sediment yield.

Major accomplishments:

Papers
Lauer, J.W. (2005), Hydrologic and geomorphic controls on the downstream transport of a wave of fine sediment, Clark Fork River, Montana,  EOS Trans., AGU, 86(52), Fall Meeting Supplement, Abstract H52A-04. 

Lauer, J.W. and G. Parker (2005), A simple model for the interaction of contaminated sediment with river floodplains,  Floodplains and Rivers: Connections and Re-Connections,  Missoula, Montana, September 22-23.