They're dangerous because waste management (sewage, drainage, etc) was dependent on the waterways for hundreds of years; only with modern infrastructure, laws a

They're dangerous because waste management (sewage, drainage, etc) was dependent on the waterways for hundreds of years; only with modern infrastructure, laws and regulations and good water management can you change it. And even then a lot of places also depend on what's happening upstream, e.g. the Rhine passes through various German industrial areas, the Nile passes through half a continent, etc. Besides that, it'll be weather dependent; if there's a dry spell and not enough water flow, toxic algae or bacteria build up.