Background
Downstream hydraulic geometry can predict several aspects of stream
properties including mean velocity and depth (Leopold & Maddock, 1953;
Lee & Julien, 2006), however, channel width is most widely studied due
to the ease of making remote or simple field measurements (e.g., Fisher
et al., 2013; Dunne & Jerolmack, 2020). Longitudinal increases in
channel width show a consistent scaling relationship with bankfull
discharge, in that the exponent (b) commonly approximates 0.5 (Equation
(1)); whereas, the coefficient (a) reflects local variations in
streambank erosivity, controlled by sediment texture, vegetation type,
flow regime, etc. (Anderson et al., 2004). Although downstream hydraulic
geometry relationships are commonly well-developed in alluvial channels,
strong downstream hydraulic geometry relationships are evident even in
regions with external non-alluvial controls, such as in bedrock channels
with local variations due to variations in bedrock erosivity (Montgomery
& Gran, 2001), and in those with discontinuous colluvial input (Wohl &
Wilcox, 2005). However, in cases where the substrate resisting forces
exceed hydraulic driving forces, downstream hydraulic geometry
relationships are not as well developed, such as in mountain streams
with very coarse sediment relative to hydraulic forces (where the stream
power to sediment size ratio (Ω/D84) < 10,000
kg/s3) (Wohl, 2004). Lakes can also contribute to
overall poor DHG relationships, with higher width to depth ratios
directly downstream of lakes, as a result of lakes trapping sediment
(Arp et al., 2007). In cases where bankfull discharge data are
unavailable, drainage area can be used a proxy for bankfull discharge
(Equation (2) & (3)) (Soar & Thorne, 2001; Faustini et al., 2009).
Because there may be additional factors to drainage area controlling
bankfull discharge, DHG relationships using drainage area tend to show
significantly more scatter, yet is still a powerful tool since only a
digital elevation model is required as input data (Faustini et al.,
2009). However, although drainage area-based DHG relationships are poor
in some regions, in a review of regression coefficients for width vs.
drainage area relationships (Eq. 3), all R2 values
exceed 0.24 and most >0.6; exponent-values (beta), which
should be less than b (commonly ~0.5) since the
relationship between drainage area and discharge (Eq. 2) where the
exponent-value (y) is commonly slightly less than 1 (Faustini et al.,
2009), mostly ranged from 0.3-0.45.
\(w=aQ^{b}\), (1)
where w is bankfull width, a is a regional coefficient, Q is bankfull
discharge, and b is a regional exponent (commonly ~0.5).
where A is drainage area, and x and y are regional coefficients and
exponents, and y is commonly slightly < 1.
\(w=\alpha A^{\beta}\), (3)
where α and β are empirical parameters and β is commonly <0.5.
Moreover, adjustment of geomorphic parameters assume an alluvial channel
where self-adjustment of channel form based on the current flow regime
is possible (Leopold & Maddock, 1953, Phillips & Jerolmack, 2016).
Therefore, semi-alluvial channels, which contain cohesive or coarse
sediment deposited by non-fluvial geomorphic processes (Polvi et al.,
2014; Pike et al., 2018; Polvi, 2021) could also contribute to
poor-fitting relationships between geomorphic and ecological
longitudinal patterns rather than fitting the established patterns from
highly connected stream networks with alluvial channels.
Geomorphic and ecological longitudinal trends are intertwined with one
another, depending on connectivity of flows and sediment in addition to
ecological meta-communities, through migration of organisms and
dispersal of seeds and propagules. Because hydrochory is dependent on
longitudinal connectivity for dispersion, the organization of passively
dispersing organisms (e.g., riparian vegetation), provides a natural
test of functional connectivity. Through hydrochory, species accumulate
downstream, which leads to higher species richness or densities
downstream (e.g., Nilsson et al., 1989, Dunn et al., 2011, Kuglerová et
al., 2015). In anthropogenically-fragmented catchments (e.g., due to
dams), hydrochoric seed dispersal is interrupted, thus altering the
biotic communities, causing proximal reaches on either side of a dam to
form dissimilar riparian vegetation species compositions (Jansson et
al., 2005). Although anthropogenic factors can create barriers in
connectivity (e.g. dams) (Nilsson, 2005; Nilsson et al., 2010) and
stream restoration has focused on increasing connectivity, many
geomorphic and ecological engineering processes serve to create natural
forms of longitudinal disconnectivity. These spatially and sometimes
temporally intermittent barriers or buffers to flow, sediment and
propagule fluxes, have received much less attention in the hydrochory
literature and how they influence metacommunity organization. Once
prevalent beaver dams and log jams cause widespread ‘leaky’ barriers
(Wohl & Beckman, 2014), serving to trap sediment, carbon, and attenuate
flows. Similarly lakes, which are widespread in northern latitudes
(Messager et al., 2016), particularly where Pleistocene glaciation has
eroded bedrock and deposited moraine dams, can have substantial effects
on stream topology and form and function of rivers where they are
connected to stream networks (Gardner et al., 2019), such as the
connectivity of sediment (Arp et al., 2007), seed dispersal (Su et al.,
2019a, b), and diversity of invertebrates (Green et al., n.d.) .
We focused our study on two catchments in boreal northern Sweden that
are heavily influenced by past continental glaciation, creating a stream
network with multiple instream lakes and coarse till deposits (Nilsson
et al., 2002; Polvi et al., 2014; Su et al., 2019a, b). The stream
networks can be divided into three process domains, defined by
Montgomery (1999) as zones with distinct geomorphic processes that
structure ecological disturbances and thus organize biotic communities:
rapids, slow-flowing reaches, and lakes (Nilsson et al., 2002; Su et
al., 2019a, b) (Figure 1a). Rapids are steep (S0:
0.1-5%) gravel- to boulder-bed channels with coarse glacial legacy
sediment (cobbles and boulders) and bedforms that do not conform to
alluvial bedform-channel slope relationships, sensu Montgomery &
Buffington (1997); slow-flowing reaches (S0 <
0.1%) are straight or meandering channels flowing through peat or fine
sediment with wetland vegetation riparian zones; lakes have inlets and
outlets and are lined with either fine sediment or coarse till. Within
our study catchments, we examined the longitudinal distribution of
process domains along nearly the entire mainstem channel, evaluated how
channel width changes downstream and examined how riparian vegetation
communities change along the channel in each new process domain. Our aim
was to determine whether these glaciated, boreal stream networks fit
established patterns of geomorphic and ecological longitudinal changes.
We test the functional geomorphic and ecological connectivity of these
catchments with high-resolution spatial data of riparian vegetation
communities and channel width.