What is a delta?

A delta is a landform that forms at the mouth of a river where it meets a body of water, such as an ocean or a lake. When a river flows toward its mouth, it often slows down and spreads out, depositing sediments it has carried from upstream. These deposits build up over time, creating a delta. The shape of a delta can resemble a triangle or fan, giving rise to its name, which is derived from the Greek letter delta (Δ). This area is typically rich in biodiversity and often serves as an important habitat for various wildlife.

The other options describe different geographical or biological phenomena that do not relate to the formation of deltas. For instance, volcanic hills are created by the accumulation of lava, while diseases affecting plants and measures of land elevation pertain to health and topography, respectively, rather than sedimentary processes at river mouths.