I think sewage is still being dumped into the Tidal basin and the reflecting pool.........which would explain why there is so much algae floating in it........


Sewage was dumped into the canal and flowed to the river onto the flats just south of the White House grounds. Because sewage accumulated on the flats as opposed to draining out to the river, the area became extremely filthy, had a foul stench, and became a breeding ground of disease. While officials debated whether action should be taken to correct this health hazard, the final straw came in February 1881 when a huge snowfall melted causing the Potomac to flood across the Mall to the National Botanical Gardens at the base of the Capitol.