A couple points to remember. A certain amount of "straying" is key to the survival of anadromous species. Without that behavior how would rivers become colonized or re-colonized. 12,000 years ago most of our rivers where buried under great thickness of ice. After the glaciers receded our rivers would have been fishless and would have remained so without this "straying" behavior.

Two - both hatchery and wild fish stray. We just tend to notice the hatchery fish more - they are more likely to be tagged/marked.

CFM -
Regarding the falls on the Cowlitz you seem to be confusing straying rates with stray contribution rates. An example, lets say the Cowlitz escapement of fall chinook was 4,000 and 1,000 were from the Lewis that would be a stray contribution rate of 25% (1,000/4,000). For the same year let's assume that the Lewis escapement was 10,000. Then the Lewis stray rate would have been 9% (1,000/(10,000 + 1,000)). I of course have no idea what the real numbers were/are and have simplified the example considerably.

Tight lines
Smalma