[3] In 1993, archaeologist Mark Hylkema documented eight different Native American sites on the Andresen Ranch along the lower North Fork Pacheco Creek, dating from 1000 B.C.

"[14] In contrast, in his 1970's PhD thesis, Professor Jerry Smith of San Jose State University observed that the North Fork above the dam has only short very warm reaches of regularly intermittent habitat unlikely to support trout.

[15] Smith cited earlier historical observer records, dating to pre-dam conditions in the 1920s, indicating the presence of trout in upper creek reaches now within Henry Coe State Park.

If the Pacheco Dam is dismantled, and not re-built, then in wet years steelhead trout could again access the upper reaches of North Fork Pacheco Creek mainstem again, although limited to 12 miles (19 km) of the North Fork mainstem's 19 miles (31 km) by the rocky barriers at the Hole in the Rock (which also render access to Mississippi Creek impossible).

When late spring flows from Anderson Reservoir on nearby Coyote Creek where withheld in the dry years 2015–2017, inability to outmigrate to saltwater resulted in extirpation of the anadromous steelhead trout population there.

[19] The importation of non-native predator fish along with San Luis Reservoir water, including non-native largemouth bass (Micropterus salmoides) and striped bass (Morone saxatilis), and/or illegal introduction of these species (typical for large California reservoirs),[20] also pose a potential threat to North Fork Pacheco Creek steelhead trout, should Pacheco Dam be re-built.

[22] Other native fish in North Fork Pacheco Creek include Monterey sucker (Catostomus occidentalis mniotiltus) and Sacramento pikeminnow (Ptychocheilus grandis).