Rivers tend to be higher in oxygen than lakes because of the mixing of water and air, and headwaters usually have more oxygen than lower reaches. Usually fast-flowing, turbulent, highly oxygenated, often rocky. Much of water resource management is involved with rate at which pollutants are removed from water column and the benthos of lakes and rivers. Molles equates lakes with small seas, a viewpoint I don't share. Lakes differ radically from seas, most notably in their isolation and oxygenation.
Lakes are a surprisingly rare phenomenon in nature.
Most were created by glacial activity. Some by major tectonic events. Reservoirs are man-made lakes. Reservoir ecology is a bit different from that in lakes, e.g., outflow in reservoirs is usually from surface, while in a lake outflow is from a greater range of depths. Many of the most famous ecologists of the early and mid-20 th century were limnologists.