3Deserts, finding water to drink won't be an issue with the glacier and some snow still in the area. Barring a Matterhorn-Whorl traverse and the East Couloir being nicer than it appeared to me, avoiding scree will be harder. I'd guess you would avoid it best if you decide to go up via Horsecreek Pass, stay right upon reaching snow at the pass and stay right to gain the ridge. Then you would be amongst brush and some rocks rather than pure, steep scree. After tagging Matterhorn Peak, you could take advantage of heading down scree to reach the normal approach to Whorl. I suppose you might head down the East Couloir of Matterhorn if you summit Whorl first: http://www.snwburd.com/bob/trip_reports/matterhorn_3.html http://www.snwburd.com/bob/trip_reports/matterhorn_2.html
I visited the same tarn ambret did on my way to Horse Creek Pass. I traversed steep, loose talus from that tarn down to the pass prior to heading up Matterhorn.
I think 3Deserts and dshoshone are referring to the smaller tarn on the south side of Horse Creek Pass (below the Matterhorn-Whorl ridge). Water was still running toward it from the slopes of Matterhorn four weeks ago. There was also snow next to it.
There is also water at Matterhorn Lake on the approach to the East Couloir. A stream will be running down to it. I even found another tarn a bit higher, nearby. All would be beautiful spots to camp near, particularly Matterhorn Lake and the area south of Horse Creek Pass, which both have spacious, soft ground.