Analysis of Flow Data from Miller Creek, Duluth, MN

Abstract

This report summarizes an analysis of flow and precipitation data for Miller Creek, a trout stream in Duluth, MN, which was undertaken in support of the MPCA-mandated temperature TMDL. The main goals of this analysis were to determine the availability and quality of Miller Creek flow data and to characterize typical summer low flow conditions to be used in subsequent stream temperature analysis. Flow data from the three existing flow aging sites (lower, middle, upper) on Miller Creek were analyzed, along with precipitation data from the Duluth International Airport. The analyses of flow and precipitation data suggest that the flow data at the lower site are relatively consistent for all years, except 2007. Flow data from the middle site for the periods 1997-2003 and 2004-2007 have different character, with the 2004-2007 data from the middle site considered suspect. Flow data from the upper site (Kohl’s) in 1997 and 1998 appear reasonable, but a rating curve does not exist to translate stage data to flow for 2003 – 2007. Relationships between stream flows and precipitation have been established at weekly timescales and are reasonable (r2 = 0.70), but with RMSEs similar in magnitude to the mean flows. Based on 1997 and 1998 data, weekly-averaged flows at the middle and upper gaging sites are, on average, 92% and 77% of the lower site, respectively. This suggests that a large fraction of the flow in Miller Creek originates from the upper portion of the watershed, upstream of the Kohl’s site. A statistical analysis of five years of flow data from the Miller Creek lower site indicates that low flows in the range of 1 to 2 cfs are quite common at weekly time scales. Therefore a rainfall event of moderate magnitude may be expected to have a significant impact on stream flow and temperature at the lower site. Although the flow record is relatively short (5 years), the results of a frequency analysis suggest that weekly mean flows near zero are possible with a 10 year return period.