The provisions of §§ 641-1 and 641-2 of this chapter shall not apply to gravel digging, washing and the processing operations or other operations when the waters of any rivers or streams within Humboldt County, from natural causes at a point immediately preceding the point where said gravel digging, washing and processing operations are being conducted, contain fifty (50) parts per million by weight of suspended solids as determined by standard methods of water analysis, or when the turbidity of the water of said rivers and streams by natural causes at a point immediately preceding the point where the gravel digging, washing and processing operations or other operations are being conducted, is more than seventy (70) parts per million as determined by a standard geological turbidity scale or the equivalent thereof. (Ord. 400, § 4, 5/2/1961)