In 1959, Union Pacific Railroad constructed a rock-filled causeway bisecting Utah’s Great Salt Lake, separating the lake into a north and south arm. Flow between the two arms was limited to two 4.6 meter wide culverts installed during original construction, an 88 meter breach opening installed in 1984, and the semi porous boulder and gravel causeway material. The south arm receives nearly all streamflows entering Great Salt Lake and a salinity gradient between the two arms developed over time. North arm salinity is often at or near saturation, averaging 317 g\L since 1966, while the south is considerably less saline, averaging 142 g\L since 1966. Ecological and industrial uses of the lake depend on salinity levels staying within physiologic and economic thresholds. Union Pacific Railroad proposed to replace aging culverts with a bridge, and provided four alternative bridge designs. Northern Utah’s variable climate complicates management of the causeway, where lake elevation and salinity are affected by wet and dry periods. Understanding the historical duration, magnitude, and frequency of wet and dry periods can inform future management decisions. I model the effect of each proposed bridge design on Great Salt Lake salinity and elevation in both arms by updating and applying US Geological Survey’s Great Salt Lake Fortran Model. I used measured historical streamflow and a 400-year tree-ring paleo-streamflow reconstruction to understand lake elevation and salinity sensitivity to longer-term climate variability. The model accurately simulates historical lake elevation and salinity and is sensitive to proposed bridge designs. Bridge alternatives vary salinity by 20 g\L within each arm using historical 1966-2012 conditions. When the model was run with the 400-year paleo-reconstructed hydrology, I find that the 20th century had the lowest average lake level of any century since 1600, and that 20th century floods were smaller than in previous centuries, both in terms of length and magnitude. With the 400-year paleo-streamflow model, differences of south arm salinity between bridge alternatives increase considerably through time, where alternative D results in salinity up to 100 g/l less than alternative A and that the current condition of the causeway would result in a fundamental change in Great Salt Lake characteristics, with the south arm approaching freshwater conditions at times. This research demonstrates that mass balance models are useful to predict management effects on terminal lake ecosystems, and provides a unique approach to reconstruct terminal lake paleo-salinity
