Ironically, your pun hits right on the head the term used in environmental science for the total impact of a discharge when combined with the flow, the load, or as a complete term, the mass loading. And sewage is bad, in the short term there are public health impacts from the gut bacteria, and the possibility of pathogens if there were infected members of the contributory population. As you note, there are also impacts from the nutrients in the sewage, especially the nitrogen which is the big factor in the Chesapeake and many coastal waters, or the phosphorus , which is the limiting nutrient in the Great Lakes. But water bodies possess what is called assimilative capacity. As the sewage moves downstream, natural processes, mainly bacterial digestion of the remaining waste, occurs in the water. This uses up oxygen, so when it is warm enough that the solubility of atmospheric oxygen is low, or the water does not have any turbulent areas to increase mechanical mixing, the rate of oxygen depletion by the putrescence process exceeds the reintroduction from the atmosphere, the dissolved oxygen levels drop, and fish with higher oxygen needs die off. If it becomes severe enough, all the fish and other aquatic organisms die off (bullheads and inverts like chironomids and some oligichaetes are able to survive very low O2 conditions.) But the decomposition of the sewage occurs in a matter of weeks, not years or centuries as for substances like the PCB's that GE has been removing from the Hudson. Zebras and Quaggas do get into this as they filter feed lots of nutrients but are not very efficient at using them, so excrete them in the areas of the beds, removing a lot of the energy that used to be associated with the pelagic area of the lake where Salmon and Alewives like to be, and concentrating it into the benthic region, where the sculpins and Lake trout should be living. In the psuedofeces of the mussels, the nutrients are not readily available to the pelagic phytoplankton , but are right where the filamentous algae like CLadophera want to grow. There is also a big knowledge void on peripheral processes, but it appears that the mussels are the chief culprit in the disappearance of the diporia, the scud like amphipod that was a major component of the food chain, and which might be a big factor in the poor returns this year. But I digress, in terms of assimilative capacity, I used to live on the Susquehanna in Binghamton, on the Endicott side of the river, and we would routinely be notified of DO NOT EAT THE FISH advisories due to sewage discharges from the Johnson City Waste Water plant, but these did not usually extend 20 miles downstream to Owego, because the river was able clean itself up in that distance. This also depends on the discharge being episodic, as eventually the assimilative capacity is reduce under constant loading, and the river turns into a sewer. And we all fought that back in the 60's and 70's which led to the Clean Water Act and the EPA (under Nixon, for all the conservatives who blame all this on wacko left wing environmentalists). OF course the Clean Water Act doesn't apply to Canada, but they have similar laws, and the Montreal incident is not expected to be perpetual.
As to farms or development, both are fine and necessary, and can be managed so there is little to no impact on water quality for streams that run through or border them, but it costs money to do this. The biggest source of sediment and pathogens in the Genesee River is farming, and programs like Conservation Reserve and Agricultural Environmental Management are working at installing Best Management Practices to reduce impacts of land spreading of cow manure, and soil erosion from cropping practices throughout the watershed.