It’d be bad. Real bad. An algae bloom of massive proportions. It has one huge issue.

Enough algae to make the rivers run green will use up enough oxygen at night to kill off fish and oxygen hungry invertebrates, starting a chain reaction of death.

Now you have a river full of dead organisms, so they start decomposing thanks to microbes. You know what many types of bacteria love? Oxygen. So they start using up oxygen, multiplying all the while. Night hits and the algae need to use oxygen, but a bunch die because there’s not enough. Now the river is full of literally hundreds, maybe thousands of tons of decomposing matter. The river largely goes anoxic (meaning there’s no oxygen) so things start dying left and right. A bunch of those bacteria can live with and without oxygen, so they use up what they can and keep on chugging without.

Now we’ve moved from aerobic respiration to anaerobic. You know what the primary byproducts of anaerobic respiration are? Organic acids and alcohols, which smell. The river begins to smell like an infected wound. It’s no longer green but deep, murky brown from the suspension of decomposing organisms. This continues until the river flushes everything out, but it kills what’s downstream as it continues until it hits the ocean, where it likely continues to kill everything in the vicinity until it becomes dilute enough.

I’m a microbiologist and worked with algae and cyanobacteria as an undergrad. Never underestimate the impact of uncountable billions of trillions of living organisms.