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Abstract.- Salmon redds and carcasses were surveyed on the mainstem Trinity River, from Lewiston Dam to the confluence with the Klamath River, during the 2015 and 2016 spawning seasons to map spawning abundance and distribution, evaluate pre-spawn mortality, and characterize redds by species and spawner origin. Redd counts were 2,162 in 2015 and 1,671 in 2016. We applied generalized additive models to the spatiotemporal distribution of unmarked and hatchery-marked spawned female salmon carcasses to apportion these redd counts by natural- and hatchery-origin Chinook Salmon Oncorhynchus tshawytscha and Coho Salmon O. kisutch. This methodology only allows for the partitioning of redds constructed by hatchery- and naturally produced females and does not account for the origin of the male spawners. In 2015, we estimated that 1,772 redds were constructed by natural origin Chinook Salmon, 331 by hatchery-origin Chinook Salmon, and the remaining 59 were attributed to Coho Salmon. In 2016, we estimated that 1,516 redds were constructed by natural-origin Chinook Salmon, 91 by hatchery-origin Chinook  Salmon, and the remaining 64 were attributed to Coho Salmon. Natural-origin Chinook Salmon spawned throughout the mainstem river while the distribution of redds constructed by hatchery-origin Chinook Salmon were highly skewed toward Lewiston Dam and the Trinity River Hatchery (83% and 87% were within 10 km of the dam in 2015 and 2016, respectively). Long-term trend analyses from 2002 to 2016 showed no significant change in the abundance of natural-origin Chinook Salmon redds constructed in the mainstem; while, the number of hatchery-origin Chinook Salmon redds decreased. The proportion of total annual natural-origin Chinook Salmon redds decreased in the reaches nearest to Lewiston Dam and increased in the mid-river reaches from 2002 to 2016 while the proportional distribution of hatchery-origin Chinook Salmon redds remained almost completely within the two reaches nearest to Lewiston Dam. Pre-spawn mortality of female Chinook Salmon was low in both 2015 (0.8%) and 2016 (0.7%).