Barriers to Dispersion of River Fishes 117 
out from it. Running back among the mountains to the north- 
ward are two small canyons down which come two small streams. 
On the opposite is another canyon down which comes another 
small stream. The extreme length of the meadow from east 
to west is about a mile, while the width from north to south 
is not much less. The larger of the streams coming in from 
the north is Pacific Creek, which, after winding along the western 
side of the meadow, turns abruptly westward, leaving the meadow 
through a narrow gorge. Receiving numerous small affluents, 
Pacific Creek soon becomes a good-sized stream, which finally 
unites with Buffalo Creek a few miles above where the latter 
stream flows into Snake River. 
‘Atlantic Creek was found to have two forks entering the 
pass. At the north-end of the meadow is a small wooded canyon 
down which flows the North Folk. This stream hugs the bor- 
der of the flat very closely. The South Fork comes down the 
canyon on the south side, skirting the brow of the hill a little 
less closely than does the North Fork. The two, coming to- 
gether near the middle of the eastern border of the meadow, 
form Atlantic Creek, which after a course of a few miles flows 
into the Upper Yellowstone. But the remarkable phenomena 
exhibited here remain to be described. 
‘Each fork of Atlantic Creek, just after entering the 
meadow, divides as if to flow around an island, but the stream 
toward the meadow, instead of returning to the portion from 
which it had parted, coritinues its westerly course across the 
meadow. Just before reaching the western border the two 
streams unite and then pour their combined waters into Pacific 
Creek; thus are Atlantic and Pacific creeks united and a con- | 
tinuous waterway from the Columbia via Two-Ocean Pass to 
the Gulf of Mexico is established. 
“Pacific Creek is a stream of good size long before it enters 
the pass, and its course through the meadow is in a definite 
channel, but not so with Atlantic Creek. The west bank of 
each fork is low and the stream is liable to break through any- 
where and thus send part of its water across to Pacific Creek. 
It is probably true that one or two branches always connect 
the two creeks under ordinary conditions, and that following 
heavy rains or when the snows are melting, a much greater 
