[5] The creek is largely channelized in the lower watershed, and includes a drop structure near the city of Brentwood that appears to be a complete passage barrier.

[6] Human activities have led to the loss of over 80 percent of suitable spawning habitat for fall-run Chinook salmon (Oncorhynchus tshawytscha) in the Sacramento and San Joaquin River watersheds.

[9] Below the dam and above the drop structure barrier, there is approximately 7 miles (11 km) suitable gravel quality, quantity, and vegetative cover to support Chinook salmon spawning.

The presence of coastal rainbow trout (Oncorhynchus mykiss irideus) has not been established in fish surveys of Marsh Creek above the drop structure.

[15] Numerous perennial pools in the intermediate and upper zones of the Marsh Creek watershed have been rumored to support rainbow trout (Oncorhynchus mykiss), however this has not been confirmed by fish sampling studies.

[5] The intermediate and upper watershed zones are also home to four federally listed faunal species – the Alameda whipsnake (Masticophis lateralis), California red-legged frog (Rana aurora draytonii), San Joaquin kit fox (Vulpes macrotis mutica), and vernal pool fairy shrimp (Branchinecta lynchi).

[5] The tidal waters at the mouth of Marsh Creek and Big Break provide habitat for the federally threatened Sacramento splittail (Pogonichthys macrolepidotus) and Delta smelt (Hypomesus transpacificus).

Big Break, where Marsh Creek enters the Delta, is one of only three locations where adult splittail still congregate in large numbers.

[15] In 2014, Save Mount Diablo (SMD), a conservation group, announced that it had bought a 51 acres (21 ha) tract of land along a 3,100 foot (940 m) stretch of Marsh Creek.