leaning on the white-washed balustrade that prevented the unwary , or unsteady , from plummeting from the brick-built bridge into the canal below , art watched two swans , attempting to steal a march on the rest of their population in the race for reproduction , gathering material with the purpose of building a large nest on a raised bank , at a point where the stream bifurcated , one fork disappearing over a low cataract , the other joining up with the course of the main river .