Concerns regarding changes in the topography of our land

Can anyone advise regarding a recent(last 3 or so months) observation of areas(round-shaped 10" to 3' in diameter and about 1 to 3 feet deep) in the property that look like the ground is sinking? This area is in the flood plane and only about 1/2 to 1 mile from the creek, but our neighbor does not have this on their land, and this has never happened before.