A study of the evolution of the Mississippi Delta reveals that a thousand years ago, even as sea levels rose, new land in the region grew at steady rates. Unfortunately, the study also shows, this resilience of the delta is greatly outpaced by modern land loss in the region. This suggests that only a small portion of the Mississippi Delta will be sustainable in future, as the world continues to warm and sea levels rise. And land loss will continue, say the authors, even if river diversions that bring land-building sediment to shrinking coastlands -- perhaps the best solution to sustain portions of the delta -- are put in place. The Mississippi Delta is a region of great economic and ecological value, but over the past century, it has experienced one of the highest rates of net land loss in the world due to multiple factors, including direct human activities that have reduced sediment delivery, as well as canal dredging and wave erosion from sea level rise. A delta's perseverance depends on the delivery of sediment to the coast via river channels; however, little is known about the long-term rates and patterns of delta growth through distributary channel activity. Here, Elizabeth L. Chamberlain and colleagues used optically stimulated luminescence dating of mouth bar sand deposits from the Lafourche subdelta in the central Mississippi Delta to determine the long-term rates of land growth in the region. This optical dating technique can determine when sediment grains were deposited by measuring their last exposure to daylight. The results show that, over a 1,000-year period, growth rates were constant (100 to 150 meters per year) producing approximately 6 to 8 square kilometers of new land each year. However, these average prehistoric rates of land growth are about five to seven times lower than the recent human-enhanced rates of Mississippi Delta land loss. The findings will inform efforts in coastal policy and management in this low-lying U.S. region, including those related to strategically placing river diversions that deliver needed sediment.