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The Swan River Oxbow (005) site is unusual in that the H. 

 a quatilis populaticxis ocojpy areas in and neeu: an old, 

 retired oxbow of the previous river channel. Ihe site is 

 physiographiccilly very different fron the glacieil pothole 

 d^ressions which the species inhabits elsewhere in the Swan 

 VeLLley. However, the bottom sediments of the sloughs eire of 

 a similar cxxTsolidated texture, and nany of the cannon 

 associated species eure present, especially Carex vesicaria 

 and Eouisetum fluviatile . 



Most sites in Spokane County, Washington, are mapped as 

 OoceLLalla silty clay loam, a poorly drained soil formed in 

 volcanic ash mixed with silty alluvium, under sedges, rushes 

 eind grasses. At least one site is mapped as Sendahoo muck, 

 a very poorly drained organic soil (Donaldsoi and Giese 

 1968) . 



The Idaho population falls within a napping unit containing 

 soils of the Harpson series, which are cocirse-silty, ndxed, 

 frigid Fluventic Haploxerolls. They are very de^, 

 moderately well drained soils on valley floors. The soils 

 are formed in alluvium derived from various sources. Slope 

 is 0-3% (Soil Conservation Service 1981) . Tliese soils 

 actually occur in adjacent bottomland meadows and are 

 generally not submerged. 



6. Dependenoe en natural disturbance: Hcwellia aouatilis is 

 restricted to aquatic habitats which typically contain water 

 for most of the growing season, but which dry out in many 

 eupeas by late sumer or early fall. The pothole ponds are 

 stable landforms which would be influenced mainly by 

 vegetational changes. However, in the case of the Swan 

 River Oxbcw (005) site in Montana, it occurs in a flood 

 plain area which is ocnpletely inundated during ^ring run- 

 off. HcMellia aquatilis appears to be tolerant of this 

 situation, as the populations return each season (with 

 variation in size) from the seed bank. The extent, if any, 

 to which the species depends on the drying of its habitat 

 each year, i.e., to promote seed germination, is unknown. 

 However, if. aquatilis may behave as a true "vernal pool" 

 species. It is suspected that any disturbance which alters 

 the local surface or subsurface hydrology euround the 

 habitats nay influence the populations. 



7. Other unusual physical features: None known or observed. 



C. Biological cheuracteristics. 



1. Vegetation physiogncnty and oomunity structure: Howell ia 

 aquatilis occurs in wetland comnunities dominated by 

 emergent vegetation. In Montana and Idaho, the ponds and 

 wetlands are typically surrounded by tenperate ocaiiferous 

 forests dominated by trees with more or less ocxiical 

 crowns. The ininediate margins of these wetlands often have 



