Over the past hundred years, most of the major rivers that flow into the Gulf of Mexico have been substantially altered in one way or another. However, the areas where rivers flow into the Gulf – ecosystems called estuaries – are vital for young crabs, shrimp, oysters and most of the species of fish people eat. Many rivers have been dammed; their water diverted for use in cities or for agriculture. As a result, some important estuaries may not have enough fresh water to maintain their function as the nurseries of the Gulf. In additon, the natural flow of water across landscapes has been blocked or altered by roads, navigation channels and oil and gas infrastructure. Changing the circulation patterns of both fresh and saltwater within coastal ecosystems can lead to habitat degradation, affecting many different types of wildlife. Hydrologic restoration projects also help maintain optimal salinities needed for the success of other restoration projects, such as oyster reef and marsh creation projects.