Weak downstream hydraulic geometry relationships
Along the mainstem of Bjurbäcken, over 1300 measurements of channel width and drainage area were made with drainage areas ranging from ~50- 360 km2, and along the mainstem of Hjuksån, ~900 measurements were made with drainage areas ranging from ~8 to 360 km2. As expected, when lake widths are included, downstream hydraulic geometry relationships between drainage area and channel width, as defined by a power relationship, are extremely poor, with one catchment even showing an overall decrease in channel width. When lakes are excluded, the relationships remain poor but weakly positive. When lakes are excluded in Bjurbäcken, located above the FHC with coarser glacial sediment, there is still a very weak relationship with an R2 of 0.08 with an exponent (β-value) of 0.34 (Figure 3). When only focusing on rapids, the R2 value increases to 0.30, and the β-value is 0.56. In Hjuksån, located below the FHC, the power relationship excluding lakes is stronger than in Bjurbäcken with an R2 value of 0.43 and a β-value of 0.68, and the relationship with only rapids even decreases in explanatory power to an R2 of 0.42 and a β-value of 0.39 (Figure 3). The two catchments show opposing directions of explanatory power for the two reach types: Hjuksån has stronger explanatory power for slow-flowing reaches (R2 = 0.57), whereas Bjurbäcken has very low explanatory power for slow-flowing reaches (R2 = 0.04).
Significant multiple linear regression models including combinations of factors of downstream distance from a lake and categorical variables of surficial geology types were found for both catchments, increasing explanatory power of width predictions, in particular in the Hjuksån catchment. Downstream distance from a lake was only a significant factor in Hjuksån (below the FHC), where increased distance downstream from a lake decreases predicted channel width. Furthermore in the Hjuksån catchment, for both rapids and slow-flowing reaches combined, the surficial geology types of coarse glacial till and bedrock lowered channel width, whereas deltaic and fine subglacial sediment increased channel width. The relationship for only rapids contained peat as the only significant surficial geology type, which increases channel width. Exponent (β-) values ranged from 0.51-0.65, and R2-values ranged from 0.47-0.69, where the relationship for only slow-flowing reaches was the strongest. In the Bjurbäcken catchment, the downstream distance from a lake was not a significant factor, but various combinations of surficial geology types were significant factors. The presence of peat or coarse till decreased the predicted width, while the presence of bedrock had opposite effects for relationships of only rapids (decreasing channel width) compared to slow-flowing reaches (increasing channel width). Even with the additional explanatory parameters, the drainage area-width relationships for Bjurbäcken were very poor with very flow R2-values, ranging from 0.10 to 0.33, where the relationship for rapids had the highest explanatory power. Exponent (β) values were much lower in Bjurbäcken, than for Hjuksån, at 0.18 to 0.26, indicating a more gradual increase in channel width with increasing drainage area.