Discussion 



All four spawning areas were remarkably similar at the various spatial scales. All 

 four spawning areas are 3'''-order basin-fed streams located in alluvial (glacio-fluvial) 

 landforms within glacial trough valleys. Observations of springs, and measured 

 groundwater inflows confirm groundwater "upwelling" at all spawning sites (FWP files; 

 Bo Stewart-USFS hydrologist personal communication; this report), a pattern identified 

 in other areas (Fraley and Shepard 1989); yet the sources of groundwater appear variably 

 influenced by adjacent valley landforms. At two study sites, upwelling appears upstream 

 of valley constrictions formed fi"om either bedrock or lateral valley morainal 

 constrictions. At the two other locations, concentrated spawning occurs in "gaining" 

 reaches where large inflows of ground-water surface over short distances and both sites 

 are located immediately downstream of seasonally intermittent "losing" reaches. In one 

 such case, groundwater surfaces as the stream begins to track against a bedrock mountain 

 slope. At a fifth site (not considered in this study), a spawning area is located adjacent to 

 two intermittent lateral valleys formed of glacial alluvium, which appear to drain 

 subsurface to the spawning area of the receiving stream. 



Bull trout spawned specifically in meandering, gravel-dominated, riffle/pool 

 channels with well-developed, mixed-forest floodplains in all four streams. These 

 streams have gentle gradients (0.009 +_0.002), display high width/depth ratios (range 

 22.1-48.3), and are characterized by point bars and other depositional features, and 

 sinuosifies >1.2 (Rosgen 2002). Based on a narrow range of substrate conditions found 

 in C-4 type spawning charmels, estimates of percent survival to emergence were likewise 

 similar with a range of 32.7 - 35.5%. 



Confirmed in other areas of western Montana (Weaver and Fraley 1991), bull 

 trout spawning areas in the Blackfoot watershed have a component of groundwater 

 upwelling. This moderating influence helps prevent fi^eezing of the egg pocket (Pierce et 

 al. 2004), and forms a stable environment necessary for embryo survival, development 

 and emergence (Thomas 2002; Weaver and Fraley 1991). Although we found no 

 difference (P=0.37) between substrate and water column temperatures, other recent 

 studies of bull trout spawning sites found mid-winter temperatures were warmer (P<0.05) 

 compared to downstream (non-spawning) areas where channels are more prone to 

 extreme (anchor) ice formation (Pierce et al. 2004). This local warming often forms 

 observable ice-ft-ee environments during the core winter months. 



Quantified morphological, thermal and visual properties of spawning areas (at 

 various spatial scales) provide a fi^amework for assessing potential (eg. historical) 

 spawning sites for similar valleys within comparable physiographic regions. The narrow 

 range other variables (eg. substrates sizes) may fiirther act as spawning site indicators in 

 lower-order streams, non-glacial valley landforms or upwelling areas found in other 

 physiographic or geomorphic settings. 



Restoration applications obviously relate not only to discrete and narrow range of 

 spawning areas properties, but also to the non-spawning spatial requirements of bull 

 trout. In this study, fluvial bull trout spawn in predictable stream environments with 

 substantial base flows (range 13-32 cfs), where connectivity, complexity and cold 

 summer temperatures create the capacity for movement, rearing and refugia. In areas 

 such as upper Nevada Creek where bull trout are (or nearly) extirpated, anthropogenic 



