Suspended Solids Carried by the Gallatin River of Southwestern Montana: II. Using Mineralogy for Inferring Sources

Abstract

Abstract Silt and clay carried in suspension by a mountain river in southwestern Montana were examined by X‐ray diffraction. Suspended sediment samples were collected from the river and its tributaries during spring snowmelt. In general the mineralogy of the sediment was related to geology of the drainage area but also differed between streams from similar geologic materials. Quartz gave the dominant diffraction peak in silts and smectite in clays from most of the streams. One tributary, Taylor Fork, was identified as the the chief source of suspended solids. The mineralogy of its silt and clay resembled that carried by the river on 8 of 10 sampling dates. Erosion of alluvial materials from the lower valley produced sediment with mineralogy similar to that presently carried by the river suggesting that the source of sediment has not changed much in recent geologic time. Branches of the tributary contributing the most sediment and of one other large tributary were examined. The branch contributing the most silt and clay to each could be identified from mineralogical measurements. An attempt to use crystallinity to determine if clays were eroded from surface soils was only partly successful.