34 



4. Riysiogr2^3hic and tcpographic c±»zu:^cteristics: In the 



Montana portion of the range, the topography of the Swan 

 Valley is of glaciad origin. Generally, the floor of the 

 vcilley is level to gently sloping, with drumlins in numerous 

 areas. Ihe pothole ponds in which H. aquatilis most often 

 ocxxirs were formed vqwn the retreat of a continental glacier 

 about 10,000 yeeirs ago. These ponds could represent 

 depressicMTS left when meisses of ice buried in outwash 

 gravels melted; they could also be formed when areas of ice 

 melted out between areas of outwash sediments which 

 accumulated xjpan the glacier surface (Alt and Hyndman 

 1986) . 



In Montana, the species is currently known to occur only in 

 the Swan River drainage, within Hydrologic Unit No. 17010211 

 as mapped by the United States Geological Survey (1980) . 



The Idaho population occurs in a mature river bottom, 

 chauracterized by a wide floodplain and a meandering river. 

 The deep, alluvial soils are derived from the erosion of 

 loess and volcauiic ash that were d^x)sited on the 

 surrounding mountains during the Pleistocene. 



In Washingtcxj, the ponds in the Spokane region are in an 

 area of basalt flows, and several of them are iitmediately 

 rijmed by basalt outcrops. The area is characterized by low 

 topographic relief (J. Gamon, pers. oontn.). 



The sites for {J. a quatilis in Montana range from 945 m (3100 

 ft.) near the south end of Swan Lake, to 1348 m (4420 ft.) 

 near the east side of Lindbergh LakB. The elevaticMTs in 

 Washington range fron 3 m (10 ft.) near the Columbia River, 

 to 707 m (2320 ft.) in the Spokane area. The Idaho site 

 occurs at 780 m (2560 ft.). 



5. E)daphic feK^ors: Howellia aquatilis is found almost 

 exclusively in ponds with bottom surfaces which consist of 

 firm, cxffisolidated clay and organic sediments. Only in two 

 cases were plants found in ponds with deeper, leirgely 

 unconsolidated bottom sediments; in these situations, most 

 H. aquatilis plants were then found in shallower areas near 

 the shore, in more oc^Tsolidated portions of the ponds. The 

 texture and depth of these bottom sediments may be very 

 iirportant in relation to seed germination requirements and 

 early grcwth of H. aquatilis . Loose, silty soil sediments 

 may lead to burial of seeds too deeply to physicadly allow 

 efficient germination and establishment. 



In Montana, the soil units which cortprise the Swan Valley 

 floor consist of Cryochrepts, Eutrctooralfs, and 

 Eutrochr^ts. The parent materials for these soils consist 

 of clayey alluvium and clayey colluvium; the resultant soils 

 cu:^ de^ (Montagne et al. 1982) . 



