Historical flow regimes and canyon bottom vegetation dynamics at Walnut Canyon National Monument, Arizona [Final report]

Rowlands, Peter; Avery, Charles; Brian, Nancy; Johnson, Heidemarie

We investigated the impact that two, upstream reservoirs, Lower and Upper Lake Mary, built in 1904 and 1941, respectively, have had on the canyon bottom vegetation at Walnut Canyon National Monument. Anecdotal and photographic evidence suggest that Walnut Canyon was an ephemeral stream with an open, rocky bed maintained by seasonal flooding prior to 1904. Today, the channel is choked with dense stands of invading, disturbance adapted, upland woody species. The Walnut Canyon drainage is not gaged and long-term hydrological data exist only for Upper Lake Mary. Three scenarios were developed using a mass-balance approach. The scenarios suggest that the reservoirs have altered the frequency of flows in Walnut Canyon and have probably altered the magnitude and duration of the flood peaks. Winter-spring flow events through Walnut Canyon have been reduced from almost an annual event to one every nine years. Most reservoir spills occur in years with above-normal snow pack and during wet years when the reservoirs are full. Spills from Lower Lake Mary have varied in duration from three to ten weeks. Summer flows from the reservoirs have been completely eliminated through Walnut Canyon. Only unusually heavy local runoff from canyon slopes and tributaries below Lower Lake Mary creates any summer flow through the monument. Magnitude of flows through Walnut Canyon during these events has been variable, according to observations, but unquantified. Estimates of annual runoff into Walnut Canyon, as predicted by our mass-balance calculations, suggests a reduction from 290,000 to 25,000 acre-feet due to the presence of the two reservoirs. Dendrochronological analysis of ponderosa pine and Arizona walnut reveal changes in the mean ring index through time but no discernible changes can be attributed to dam construction. A 1973 vegetation survey of Walnut Canyon reported very similar plant species composition and cover as that seen in the canyon bottom today. After a fortuitous, eight-week spill in 1993, we remeasured eight, existing permanent monitoring plots, established in 1989, and compared plant species cover. Short-lived perennial and annual plant species not adapted to flooding and disturbance were physically eliminated, but have since recovered. Invading upland tree species, such as Rocky Mountain juniper, were removed from the channel proper, but seedlings and sprouts reestablished after the flood. Disturbance-adapted, upland woody species, such as New Mexico locust and Gambel oak, were actively resprouting. Boxelder and Arizona walnut appear to be slowly declining due to limited flood-induced reproduction. Comparisons of 1993 with 1989 plot data showed that the flood of 1993 brought about little overall change in plant species cover or composition. In contrast to Walnut Canyon, nearby undammed ephemeral drainages subject to twice-yearly seasonal runoff, maintain open channels and support obligate riparian plant species, such as willows. This suggests that a true riparian community with obligate riparian plant species may have been gradually eliminated from Walnut Canyon since 1904. The canyon bottom vegetation in Walnut Canyon appears to be in perpetual succession or transition. It is in dynamic equilibrium with post-dam, irregular spill events.