List these in order of size tributary, stream, rill, gully, runoff, river.

A runoff-water refers to the flow of water after a stormwater, or other source. It occurs because the soil is saturated, it's at full capacity of water absorption and the soil drains the excess. That's why is the first, because it is the smallest among the options.

A rill refers to a superficial channel cut into soil caused by soil erosion, it doesn't half too much depth (a few centimetres), so it cannot have a great amount of water, just more than a runoff-water. A rill is like cracks in the soil.

A gully is when running water erodes the soil, creating a landform, it typically occurs in hills, there it's created easier. Gullies can create ditches or small valleys, that's why it has to have the third place.

A stream is a small river, it's a surface water that flows into a channel. Basically, streams are the result of geological and hydrological factors, they are part of the water cycle.

A tributary(or affluent) is a river that flows into a lake or other main stream rivers, but it doesn't flow into the sea, that's the difference. A tributary is also call an affluent. So, the main characteristic is that flows into lakes or bigger streams, the opposite is a distributary, which flows away from main streams, that is, in the opposite direction of tributaries.

A river is a natural flowing water which can flows towards lakes, oceans, seas or other main stream rivers. The difference with tributaries is that rivers flow into seas or oceans, and they can have a bigger mass of water.

Therefore, the right order according to size is: Runoff, rill, gully, stream, tributary, and river.