Dynamics of Gravity Currents in the Chicago River, Illinois Carlos M. García , Kevin Oberg , and Marcelo H. García 1 Instituto Superior de Recursos Hídricos, Universidad Nacional de Córdoba, cgarcia2mjc@gmail.com 2 U.S. Geological Survey, Office of Surface Water, kaoberg@usgs.gov . 3 Ven Te Chow Hydrosystems Laboratory, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, mhgarcia@uiuc.edu Abstract During the winter months, the Chicago River in Chicago, Illinois, USA, is subject to bi- directional flows, and gravity currents are thought to be responsible for such flow behavior. The occurrence, frequency, and evolution of gravity currents in the Chicago River have been described using a unique set of field observations made using an upward-looking acoustic Doppler current profiler (ADCP) during the period November 20, 2003 to February 1, 2004. Detailed field observations performed on December 2005, based on both ADCP and water- quality measurements, are employed to validate hypotheses about the characteristics of the currents and the source of the denser water. Observations indicate bi-directional flow is a common condition in the Chicago River system, and that density differences driving the gravity currents are mostly due to salinity differences between the North Branch and the main stem of the Chicago River, whereas temperature differences do not seem to affect the creation of gravity currents. 1. Introduction The present-day Chicago River (hereafter CR) flows west from Lake Michigan, through downtown Chicago, and joins flow coming from the North Branch of the Chicago River (hereafter NB) where it enters the South Branch of the Chicago River (hereafter SB). Flow in the CR is controlled by the Lockport Powerhouse and Controlling Works (near Joliet, IL) and by the Chicago River Controlling Works (hereafter CRCW) and the Chicago Lock. During the summer, water from Lake Michigan flows into the CR through sluice gates in the CRCW and, because of lockages, through the Chicago Lock at CRCW. Flow from Lake Michigan into the CR during the summer months, called discretionary diversion, is used to improve the water quality in the CR and the Chicago Sanitary and Ship Canal (hereafter CSSC). This has been the case since the reversal of the Chicago River in the early 1900’s, in order to protect the main water supply for Chicago (i.e. Lake Michigan). During the winter, flow from Lake Michigan into the CR is small, when compared to the summer, and typically results from leakage through the gates and sea-walls at CRCW and some sporadic lockages. Other contributions to the CR discharge include water from direct precipitation and discharges of water used for cooling purposes from neighbouring buildings. The NB carries runoff from the watershed up-stream and treated municipal and industrial wastewater released by a water- treatment plant located 16 km upstream from the confluence of the branches. The treated wastewater effluent accounts for as much as 75 percent of the discharge in the NB during the winter months (Manriquez et al., 2005). Beginning in 1998, the U.S. Geological Survey (USGS) observed bi-directional flow conditions while making discharge measurements in the CR. Although the duration of this bi-