Mountains play an important role in providing water resources from snow and ice to downstream urban population centers. In Utah, USA, nearly 86% of the state's population resides in the rapidly growing urban corridor along the Wasatch Front. Water along the Wasatch Front is stored in several deep reservoirs in the Provo River Watershed. Additions of nutrients, into these reservoirs, may stimulate the growth of primary producers (e.g., total phytoplankton and cyanobacteria) potentially attenuating water quality. To determine the influence of nutrients on primary producers in the Provo River's reservoirs, identify whether cyanobacteria are transported downstream, and quantify nutrient thresholds that regulate phytoplankton responses, we monitored and experimentally manipulated total phytoplankton and cyanobacteria in rivers and reservoirs. We found that total phytoplankton biomass (measured as chlorophyll a concentrations) was correlated with the total phosphorus (TP) in the Jordanelle Reservoir and the outflow from the Jordanelle Reservoir. Cyanobacteria biomass (measured as phycocyanin concentration) was correlated with dissolved inorganic N (DIN):SRP in the Deer Creek Reservoir and river sites below the Deer Creek Reservoir. Based on next generation sequencing 16S rDNA of all waters evaluated, the relative abundance of cyanobacteria within bacterial communities was extremely low, with the highest relative abundance of cyanobacteria present in the reservoirs being 10.7% for Deer Creek and 5.5% in Jordanelle during the late summer. Of the 25 genera of cyanobacteria that were found across all waters, five species have the ability to produce cyanotoxin: Microcystaceae; Aphanizomenon MDT14a; Aphanizominon NIES81; and Planktothrix NIVA-CYA. Season determined bacterial community composition in the river and reservoir over the almost two years of sampling with bacterial communities being distinct between the limnetic location in Deer Creek Reservoir to the outflow into the river immediately below the dam. We found no difference between the bacterial communities in the limnetic zone in Jordanelle and the river site directly below Jordanelle Dam. In the nutrient starvation bioassays, cyanobacteria and total phytoplankton responded to the nutrient additions, but there were no specific nutrient thresholds where pigment concentrations leveled out even as nutrient concentration increased. However, when P was added as SRP treatments both total phytoplankton and cyanobacteria concentrations increased, especially in the highest SRP treatment (0.08 mg/L) without any N addition. The addition of N alone did not influence total phytoplankton until there was at least 0.2 mg/L of P added. Further, cyanobacteria required at least 0.2 mg/L of SRP before responding to N levels above 0.8 mg/L. Thus, a nutrient threshold to maintain the reservoir at its current state would be 0.2 mg/L P and between 0.3 and 0.8 mg/L DIN. Our results identify that the water within the reservoirs and rivers are extremely clean and is in no immediate risk of extensive total phytoplankton or cyanobacterial blooms with P being the dominant driver of primary producer activity.