A flood control dike is a lengthy wall or embankment designed to keep a river route from flooding. A berm is a flat stretch of land, elevated bank, or terrace that borders a river and is used to prevent flooding. The term is also applied to similar features on lakes and estuaries. In urban areas, where space is at a premium, berm walls are built instead. The word "derive" in its verb form means "to take your source of income". That is, if the only source of income for a family was a farm, then the would be required to give up farming and rely entirely on the income from one crop per year. If this crop were to fail, then the family would be ruined because there would be no way to pay for their expenses. At the end of every season, farmers determine how much profit they have made by comparing what they spent money on fuel and other inputs like fertilizer and seed with what they were able to sell their product for. If it turns out that they didn't make enough money, then they will need to invest more time or resources into the farm business next season. In short, dikes protect villages from floods; berm walls protect farms from floods. Dikes can be either natural or artificial.