We have yet to spend much time on the actual cross-sectional shape of the channel. In the lecture you learned that in meandering streams the shape of the channel changes depending on which part of the stream you look at. The cross-sectional shape is symmetric in the straight reaches and asymmetric in the bends. See figure 1. Keep in mind that the figure is vertically exaggerated.
Rivers are usually much wider than they are deep.
So now we know what the cross-sectional shape of the meandering river is at different positions. Well, let's think about this. We know from experience that discharge is variable. Let's think our way through this question. We can guess from channel shape that width would increase faster than depth.