Swans in a River to Pass the Winter 天鵝在河中越冬

I surfed the internet and found an area of swans gathering to pass the winter. It's a river called Gongyo, not a lake or pond.

I came to the river on a Saturday this month. The width of the river is only 20 or 25m. As you see, the river is very shallow because all weirs for irrigation were put down after rice harvesting, in addition, rainfall in winter is very little.  

A young one fluttering its wings looks comfortable.

"Unwearied still lover by lover.

They paddle in the cold

Companionable streams or climb the air;

Their hearts have not grown old;

Passion or conquest, wander where they will,

Attend upon them still." 

W. B. Yeats

I know there are lots of small fishes in the river, but swans usually eat waterweed.This riverbed is gravel not mud. I am curious to know how they get foods here. After coming back home, I read the city website saying: the first swans came here 14 years ago, every year flocks of swans come to this river in the end of October and fly back in the end of March and 220 swans passed the last winter in this river.