Seasonal change of fine-grain sediment deposits near three tributary confluences in Oak Creek, Arizona

Discrete, fine-grain sediment deposits in a high gradient stream are dynamic features that are continuously affected by discharge and sediment supply. Fine sediments in the bed of Oak Creek, Arizona are known to act as a reservoir for organic pollutants including Escherichia coli, which is an indicator of fecal contamination and the potential presence of associated enteric viruses and other pathogens. Fecal material in Oak Creek originates from sources throughout the watershed. Disturbances to creek sediments such as roiling or dredging can release high concentrations of fecal contamination into the water column. A study of fine sediment deposits in Oak Creek will add to the body of information available to help resource managers mitigate fecal pollution in the watershed.;A three year study of fine sediment deposits ( ≤ 2.0 mm) in Oak Creek using cross-sectional surveys to map sediment bars was carried out at three field sites near tributary confluences following spring and summer high flows from 2012 to 2014. Survey data showed seasonal patterns of summer aggradation and winter erosion. Analysis of samples collected in the field found that fine sediments were composed of sand, silty sand, and sandy silt sized particles with organic content estimates ranging from 2 to 7 percent. Sediment volume estimates for field sites were calculated using data from field surveys, laboratory analyses, and GIS analyses. Fine sediment volume trends appear to be linked to the magnitude and extent of seasonal streamflows related to the bimodal precipitation regime. Elevated streamflows generated by spatially limited summer convective storms were smaller compared to winter and spring streamflows produced by cyclonic/frontal storms and snowmelt. Ephemeral tributary streams are thought to provide a significant amount of fine sediment to the mainstem stream when summer localized convective storms generate streamflow within particular subcatchments. Erosion of fine sediment deposits occurs during winter and spring high flows that can even remove sediment bars altogether in some high discharge events. A wildfire that occurred near the end of the study in May of 2014 affected about 7.1 % of the upper watershed. This event is likely to increase the volume of sediment and woody debris supplied to the stream with associated modifications to stream morphology, though no immediate changes were detected during fall 2014 surveys.