The following website provides details about the historic and current distributions of the CA Freshwater Shrimp:
link to Sacramento Fish and Wildlife Office .

Unfortunately, it may be exceedingly difficult to obtain true population estimates for small invertebrates such as shrimps. As a proxy for estimates of actual population sizes, many conservation scientists instead monitor the distribution and habitats of a species, as well as their relative densities within each habitat or portion of their geographic range. For instance, for the shrimp, there are several distinct watersheds, or river systems, where the shrimp used to occur historically.

Scientists can develop an accurate assessment of the shrimp populations health and its trends (whether the population is increasing or decreasing) by monitoring many different portions of each watershed over time and estimating the number of shrimp at each monitoring site, as well as by monitoring the overall habitat health of their environment.

On a website I found information that indicates that the shrimp historically inhabited lowland, year-round (perennial) streams throughout three counties in California: Marin, Napa, and Sonoma. In the 1930s, a close cousin shrimp in southern California had already gone extinct. When biologists first studied the shrimp, they were only found in nine streams, and by the late 1970s, the CA Freshwater Shrimp could only be found three streams. The shrimp was extirpated from six streams during this period due presumably to channelization of streams, water withdrawal from streams, pollution, introduced predators, and lining streams with concrete for flood control. However, since the shrimp was federally listed as endangered in 1988, it has undergone a tremendous comeback and now inhabits a total of 16 different streams, up from a low of only three streams three decades ago.