Diamond Pond, Harney County, Oregon: vegetation history and water table in the eastern Oregon desert

Cores obtained in 1978 from Diamond Pond, Diamond Craters, Harney County, Oregon, as part of the Steens Mountain Prehistory Project, provide a record of vegetation change on the sagebrush/shadscale ecotone and of local and perhaps regional water tables. Pollen, macrofossils, sediments, and charcoal from these radiocarbon-dated cores were analyzed. Varying abundance ofjuniper, grass, sagebrush, and greasewood pollen, and of aquatic to littoral plant macrofossils reflects changing regional effective moisture and local water table since 6000 B. P. Eleven dates spanning 5200 radiocarbon years and four regionally correlated volcanic ashes establish the dating of seven periods of diflferent moisture regimes; 1. Greasewood and saltbush pollen dominance before 5400 B.P. indicates shadscale desert. Rapid accumulation of alternating silts and medium sands lacking aquatic plant macrofossils and pollen reflects periods of ephemeral ponds with water table 17 m below the present level and considerable erosion of maar slopes, 2. Increasing sagebrush pollen from 5400 to 4000 B. P. indicates sagebrush expansion into shadscale desert. Scirpus, Riimex, Ceratophyllum. and Polygonum pcrsicaria macrofossils and finely laminated clayey silts evidence perennial pond. 3. From 4000 to 2000 B.P. abundant juniper and grass pollen reflects extensive juniper grasslands (juniper seeds from trees growing nearby fell into the pond during this period). Rising charcoal values indicate greater importance of fire. Deepest late-Holocene pond ca 3700 B.P. corresponds with postulated intensive human occupation of northern Great Basin marsh and lake locales. 4. Between 2000 and 1400 B.P. increased sagebrush pollen mirrors reduced effective moisture and reexpanding sagebrush steppe. More abundant Scirpus and Rumcx macrofossils evidence shallow pond. 5. From 1400 to 900 B.P. more numerous grass pollen indicates returning greater effective moisture resulting in deeper water with abundant Potainogeton. 6. About 500 B.P. increased greasewood and saltbush pollen evidences drought. Ruppia seeds and pollen and the mollusk Musculium indicate shallow, brackish water. 7. Abundant juniper and grass pollen reflects moister conditions between 300 and 150 B. P. Numerous Ceratophyllum fruits indicate deeper, freshened water. Since the mid1800s man and changing climate have encouraged sagebrush reexpansion. Increased Scirpus macrofossils indicate shallower water. Although topographic diversity creates the of Malheur and Harney lakes during the past variety of habitats found in the Great Basin, 160 years. volcanic eruptions, tectonic activity, and Diamond Pond in the Diamond Craters sharp variations in climate, lasting 100 to 200 area of the southern Harney Basin, Oregon, is years, have affected the size and diversity of an ideal location to watch changing vegetation these habitats (Mehringer 1986, 1977). Clipatterns for three reasons. First, Diamond matic change in the Great Basin, especially in Pond lies adjacent to and at the same elevation the north, is best reflected in its fluctuating as Diamond Swamp (Figs. 2, 3), and its water lakes and marshes (Mehringer 1986). level is controlled by groundwater discharge. Since early in 1826 when Antoine Sylvaille Second, varying abundance of aquatic and litand five other trappers entered the Harney toral plant macrofossils reflects the expansion Basin ofsouthern Oregon (Fig. 1) and reached and contraction of the fringe of littoral and the river later named for him—the Silvies emergent aquatic species that presently surRiver (Rich etal. 1950)—the volumes of Mairound Diamond Pond (Fig. 3). Finally, heur and Harney lakes have varied considerchanges in the shadscale, lower sagebrush, ably (Piper et al. 1939). Accounts from early and juniper communities that adjoin on portrappers, descriptions by early settlers, and tionsofDiamond Craters (Fig. 2) are reflected records maintained by local, state, and federal in the detailed microfossil record of vegetaagencies reveal dramatic changes in the levels tion change unparalleled in the northern Department of Anthropology, Washington State Universit\ , Pullman, Washington 99102.