The livelihoods of millions of people who live in river deltas, among the world's most productive lands, are at risk. Created where large rivers meet the ocean and deposit their natural sediment load, river deltas are often just a few meters above sea level. And while they make up less than 0.5% of the world's land area, river deltas contribute more than 4% of the global GDP, 3% of global crop production, and are home to 5.5% of the world's population.