This data release contains seasonal source-specific estimates of total nitrogen (TN) and total phosphorus (TP) loads from watersheds draining to Washing water of the Salish Sea (Puget Sound region,12,314 e2NHDPlusV2 reaches plus 2 added transfer reaches) using a dynamic SPARROW (Spatially Referenced Regressions on Watershed attributes) model. Input data, output predictions, model control files, and model source code are provided to fully reproduce results. The modeled period was from January 2005 through December 2020 in seasonal time steps, or 64 periods, and the following seasonal definitions were applied: winter includes January, February, and March; spring includes April, May, and June; summer includes July, August, and September; and fall includes October, November, and December.