33 CFR §333.35

1. (a)The District Engineer will not undertake new scientific and technical research to inform their analyses unless that is essential to a reasoned choice among alternatives and the overall costs and time frame of such undertaking are not unreasonable. Rather, the District Engineer will make use of reliable existing data and resources.
2. (b)When the District Engineer is evaluating an action's reasonably foreseeable effects on the human environment, and there is incomplete or unavailable information that cannot be obtained at a reasonable cost or the means to obtain it are unknown, the District Engineer will make clear in the relevant environmental document that such information is lacking.