Lake Bonneville began to rise from elevations similar to those of modern Great Salt Lake about 30,000 years ago. During its transgressive phase in the closed basin, lake level oscillated because of changes in climate but the lake gradually rose until about 18,000 years ago when it reached its highest elevation, marked by the Bonneville shoreline. At that level the lake had risen to the lowest point on its basin rim and had begun to overflow into the Snake River drainage near Red Rock Pass in what is now southeastern Idaho. The overflow, which would have begun as a trickle across the dam formed by the Marsh Creek alluvial fan, quickly evolved into a tremendous flood, the Bonneville flood, which charged down the Marsh Creek valley to the Portneuf River, into the Snake River and then into the Columbia River and Pacific Ocean. Groundwater sapping on the north slope of the Marsh Creek alluvial fan, which began long before the lake had reached its highest level, added to the instability and ultimate collapse of the fan-dam. The Bonneville flood probably lasted less than a year, during which time almost 5000 km3 (1200 mi3) of water flowed out of the lake basin with a maximum discharge of about 1 million m3/s (35 million ft3/s). Downcutting during the flood through the Marsh Creek alluvial-fan deposits and into the underlying Neogene sand, mud, and landslide debris, caused lake level to drop about 130 meters (425 ft). River flow from the lake across the Red Rock Pass threshold and out of the lake basin continued non-catastrophically for about 3000 years after the flood ended; the Provo shoreline formed during this overflowing phase. The Provo shoreline is distinguished from other shorelines of Lake Bonneville by its topographic position, strong development, and thick accumulations of tufa. At the end of the overflowing phase, about 15,000 years ago, climate change and a shift to a negative water balance (more water evaporated off the surface of the lake than entered by rivers or direct precipitation) caused the lake to return to its closed-basin status as it declined to lower levels during the regressive phase. By 13,000 years ago the lake had fallen to an elevation similar to the average elevation of modern Great Salt Lake. During the regressive phase lake level declined approximately 200 meters (~650 ft) in about 2000 yr because of a change to warmer and dryer climate (200 meters is roughly 2/3 of the maximum depth of Lake Bonneville). Although Lake Bonneville and Great Salt Lake are collectively one lake system, the name Lake Bonneville is applied to the lake during the period from 30,000 to 13,000 years ago, and the name Great Salt Lake since 13,000 years ago.