For over a century, the Mississippi River has not created many any large offshoots, because the channel was mostly constrained behind levees. This changed in 2020, when a small channel about 50 miles southeast of New Orleans rapidly expanded. This system, now called Neptune Pass, has a flow that is about the size of the 10th largest river in the country. This talk describes how, Neptune Pass formed, how it is building a delta, and what it means for safe navigation in the Mississippi River.