With adequate flow the pollution gets diluted. But, people live on the river banks and use the river as a toilet. It drains the storm drains of a lot of Denver as therefore gets everything from oil to fertilizers to treated effluent. But if you need a quick fix you need a quick fix. Union might be a bit cleaner than Confluence. Clear Creek near Golden (WW Park or Lower CC) would be a lot cleaner (though still not clean) water.