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Salmon redds and carcasses were surveyed on the mainstem Trinity River, California from Lewiston Dam to the confluence with the Klamath River, during the 2017 spawning season to map spawning abundance and distribution, evaluate pre-spawn mortality, and characterize redds by species and spawner origin. The total redd count in 2017 was 1,982. We applied generalized additive models to the spatiotemporal distribution of unmarked and hatchery-marked spawned female salmon carcasses to apportion 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 natural-produced females and does not account for the origin of the male spawners. We estimated that 1,600 (95% c.i.: 1,435–1,762) redds were constructed by natural-origin Chinook Salmon, 348 (95% c.i.: 186–513) by hatchery-origin Chinook Salmon, and the remaining 34 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 was highly skewed toward Lewiston Dam and the Trinity River Hatchery (about 59% were within 10 km of the dam). Pre-spawn mortality of female Chinook Salmon was 1.8% for carcasses observed in all reaches and 2.0% within an intensively managed ‘restoration reach’, which is a focal area for habitat restoration improvements being implemented by the Trinity River Restoration Program (TRRP). Long-term trend analyses from 2002 to 2017 showed no significant change in the abundance of natural-origin Chinook Salmon redds constructed in the mainstem Trinity River, 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 reaches farther downstream from 2002 to 2017, while the annual component of hatchery-origin Chinook Salmon redds remained almost completely within the two reaches nearest to Lewiston Dam.