(issue was whether government’s experts’ opinions regarding whether river branch was “water of the United States” should be excluded; defendants were charged with violating Clean Water Act, making false statements, and obstruction of justice in discharge of hog waste case; issue was whether expert testimony would assist the jury in its determination; court held that expert was not permitted to “opine” as to whether river branch was “water of the United States” or whether it possesses “significant nexus” with nearby waters; both terms are legal conclusions and would “impermissibly invade” province of jury by telling jury what result to reach).