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PEARSON, KRAMER, FRANKLIN: MACROINVERTEBRATES 161 Island Park The gradient at this station was relatively low, and the sub- strate was mostly gravel with numerous clumps and snags of plant debris. Seasonal water-level fluctuations were great and bank erosion was severe which greatly reduced the stability of the sand-silt areas. Forty-seven forms of bottom fauna were collected (Table 3). The four most abundant organisms on rubble-debris substrates were Chironomidae, Baetis sp. I, Ephemerella inermis, and Trichory- thodes minutes. Maximum observed numbers were 3,949; 1,840; 829; and 334/m'-, respectively. On sand-silt bottoms, Chironomidae larvae, Ceratopogonidae larvae, and Oligochacta were the most abundant forms, reaching maximum densities of 237, 215, and 129/m2. MACROINVERTEBRATE DISTRIBUTION: 1967 (FLAMING GORGE DAM TO TAYLOR FLAT) The bottom-fauna community on rubble-gravel substrates at Little Hole was again dominated by Oligochaeta, Chironomidae, Simuliidae, and Baetis sp. I in 1967 (Table 4). Numbers of oligo- chaetes and chironomids were slightly lower in 1967 than in 1965, but numbers of Simuliidae and Baetis were greatly reduced. Density of Simuliidae was 1,022-20,261/m2 in 1965 and 22-990/mz in 1967 (Table 4). Numbers of Baetis per square meter were reduced from 721-40,124 in 1965 to 75-1,033 in 1967. Baetis sp. I had two generations per year in this stretch of the Green River. The winter generation hatched in the fall, over- wintered as nymphs, and emerged in June-July of the following year. The summer generation hatched in July, matured rapidly, and emerged in September to lay eggs which hatched and became the winter generation. Mature nymphs of the winter generation were longer (6.1-6.3 mm) than mature nymphs of the summer generation (5.0-5.3 mm). This size dimorphism between generations is common in Baetidae and provided a means of distinguishing mature nymphs of the two generations. Early in the summer of 1967, numbers of mature winter- generation nymphs were nearly equal to those estimated in 1965. Emergence, mating, and oviposition were observed in 1967, but it soon became apparent that very few eggs had hatched by late August. Nearly all nymphs taken in August were stragglers r