in Thoreau’s Journal:

We are gliding swiftly up the river by Lee’s bend. The surface of the water is the place to see the Pontederia from for now the spikes of flowers are all brought into a dense line––a heavy line of blue a foot or more in width––on both sides of the river. The pontederias are now in their prime––there being no withered heads, they are very freshly blue. In the sun when you are looking west they are of a violaceous blue….In many parts of the river the pickerel weed is several rods wide––its blueness akin to the misty blue air which paints the sky….The border of pontederia is rarely of equal depth on both sides at once––but it keeps that side in the meander where the sediment is deposited––the shortest course which will follow the shore…..This is the longest line of blue that nature paints with flowers in our fields––though the lupines may have been more densely blue within a small compass––  Thus by a natural law a river instead of flowing straight through its meadows––meanders––from side to side––& fertilizes this side or that & adorns its banks with flowers.