Distribution of Benthic Invertebrates in the Lost Streams of Idaho

The Lost Streams of Idaho (Big Lost and Little Lost rivers; Birch, Medicine Lodge, Beaver and Camas creeks) constitute a unique set of isolated lotie environments. The streams are all similar in size (discharge 1.5-2.5 mVsec) except for Camas Creek which is 3-4 times larger. Likewise, chemical conditions are similar in all streams except Camas Creek which has generally lower values. The Lost Streams are well-buffered and rich in calcium bicarbonate (100-250 mg/liter as CaC03). Sixty-one invertebrate taxa were found in the comparative collections, but only 10 (excluding Chironomidae) occurred in all six streams: a beetle (Optioservus quadrimaculatus), two dipterans (Simulium sp., T?pula sp.) three mayflies (Baetis tricaudatus, Ephemerella grandis, ?. inermis), two stone flies (Acroneuria pacifica, Isopvrla fulva), and two caddis flies (Brachycentrus occidentalis, Hydropsyche sp.). Examination of invertebrate collections from both sides of the 80-95 km wide Snake River Plain suggest that the faunas were established before isolation of the Lost Streams during the early Pleistocene. Introduction The Lost Streams of Idaho constitute a unique set of isolated lotie environments located along a 146-km front in the Upper Snake River Valley of eastern Idaho (Fig. 1). The streams originate in the Rocky Mountains bordering the Continental Divide and flow in a southeasterly direction to the edge of the Snake River Plain. The Snake River Plain is a high plateau built up of basalt lava flows that erupted