Phytoplankton growth and microzooplankton grazing rates are well studied in marine systems and lakes; however, much less is known about these dynamics within fully freshwater reaches of large rivers. The Columbia River (CR) discharges the largest amount of freshwater into the northeastern Pacific Ocean and is located in one of the most extensively managed river basins in the world (Columbia River Basin). Our study is the first experimental assessment of phytoplankton growth rates and microzooplankton grazing rates in the lower CR and provides novel insights into the dynamics of the CR planktonic food web. We undertook a two-water-year (2023-2025) study to measure phytoplankton standing stock in the field and simultaneously measure intrinsic phytoplankton growth rates and microzooplankton grazing rates within the lower CR in Vancouver, Washington, USA by employing monthly “dilution experiments” (Landry and Hassett 1982). Surface water chlorophyll measurements have been taken from this site monthly since 2005. We found the seasonal pattern of chlorophyll concentration to be similar between both time periods (2005-2023 and 2023-2025), with peaks in spring and lower concentrations in the winter months. However, the average annual chlorophyll concentration was significantly lower in 2023-2025 (3.10 μg/L) compared to the long-term dataset (5.57 μg/L). We found that average monthly chlorophyll concentration is typically positively related to increases and decreases in the phytoplankton intrinsic growth rate. From October 2023 to September 2025, the phytoplankton intrinsic growth rate ranged from –0.26 to 0.83 d-1 and microzooplankton grazing rates ranged from –0.690 to 0.729 d-1. However, we found that in most months, microzooplankton grazing rates on phytoplankton were very low and not significantly different than zero. These low and sometimes negative grazing rates could have impacts on higher trophic levels, as energy from primary production may not be transferred to larger grazers. Our findings help to advance our understanding of the planktonic food web and possible higher trophic level impacts on larger grazers by limited grazing by microzooplankton in the lower CR, as well as show the advantage of multi-year sampling programs to illustrate seasonal patterns of phytoplankton growth and microzooplankton grazing rates within a large river system.