Unfortunately, Washington's farmers have gotten the legislature to enact a law protecting them from suits for creating a nuisance. It's RCW 7.48, and reads, in part:

>(1) Notwithstanding any other provision of this chapter, agricultural activities conducted on farmland and forest practices, if consistent with good agricultural and forest practices and established prior to surrounding nonagricultural and nonforestry activities, are presumed to be reasonable and shall not be found to constitute a nuisance unless the activity or practice has a substantial adverse effect on public health and safety.

(2) Agricultural activities and forest practices undertaken in conformity with all applicable laws and rules are presumed to be good agricultural and forest practices not adversely affecting the public health and safety for purposes of this section and RCW 7.48.300. An agricultural activity that is in conformity with such laws and rules shall not be restricted as to the hours of the day or day or days of the week during which it may be conducted.

This doesn't mean they get off scot free. Their operations have to be consistent with good agricultural practices. And if your neighbor is polluting a creek, he's not exempt. I'd suggest you collect several water samples from the creek, at different times and locations along and downstream from his operation. If you can afford to have a lab test them yourselves, do it. If not, ask the county to do so. If this is really a salmon stream, you might consider contacting an environmental group like the Wild Fish Conservancy and asking whether they're interested in helping.

Or, another approach would be for you to take up falconry. Birds of prey just love fresh chicken.