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.