We identified five reaches on two streams in the Kickapoo River
watershed for this study: Billings Creek Restoration Site (BRES),
Warner Creek Upstream Site (WUP), and Warner Creek Downstream Site
(WDN) were slated for restoration in 2019, while Billings Creek
Reference Site (BREF) and Warner Creek Middle Site (WMD) would serve
as reference reaches. Both creeks feature streambeds with small
cobble and some silt, with portions partially shaded by riparian
woody vegetation. We planned to collect and compare geomorphology,
habitat, and CH4 and
CO2 flux data pre- and post-restoration on
BRES, WUP, and WDN, as well as comparing CH4
and CO2 fluxes at BRES, WUP, and WDN against
BREF and WMD through the 2019 growing season (May to October).
During the study period (May 2019 to October 2019) all reaches
experienced multiple flash flood events, with discharge varying over
one order of magnitude on the closest USGS gage (Kickapoo River at
La Farge, USGS: 05408000). Due to heavy precipitation throughout the
2019 season, construction was delayed indefinitely on WUP and WDN.
Construction was delayed and then quickly completed in July 2019 at
BRES, which included removal of woody vegetation, resloping of
streambanks, installation of in-stream habitat structures,
installation of rock riprap, and the addition of topsoil and native
prairie grass seed on top of rock riprap. The largest flood event of
the 2019 season (on 19 Jul) moved through BRES two days after
project completion.
We measured discharge weekly or biweekly at BRES, BREF, WUP, and WDN
with a SonTek Flowtracker ADV (San Diego, CA, USA). During the study
period (May 2019 to October 2019) discharge ranged from 0.63
m3/s to 2.24
m3/s on Billings Creek and from 0.54
m3/s to 1.34
m3/s on Warner Creek and both streams
experienced multiple overbank floods. These in
situ measurements include the 19 July 2019 flood event on
Billings Creek, when we were able to capture the falling limb of the
flood event once water receded to top-of-waders height, but not on
Warner Creek because of the dangers of data collection during
floods.
We measured 18 cross-sections every 30 m through the full 510 m of
BRES. Cross-sections extended 20-40 m from the leveling off point in
the river left floodplain, through the river channel, and beyond
bankfull until the ground leveled out in the river right floodplain
and were marked at their ends with semi-permanent wooden stakes. The
two upstream cross-sections (000, 030) were excluded from the study
because the channel was dramatically altered by the addition of a
permanent streamside access road during restoration construction.
Pre-restoration, 18 cross-sections were surveyed with a Real-Time
Kinematic (RTK) rover and base station (GeoMax Zenith25 Pro GNSS/RTK
Professional System, Widnau, Switzerland) supplemented with a laser
level base station (Topcon RL-H5B Self Leveling Horizontal Rotary
Laser, Livermore, CA, USA) with receiver attached to a leveling rod.
Each cross-section was surveyed for bed elevation, from the
floodplain through the channel, noting bankfull and edge of water,
with measurements taken at every change in slope and not more than
0.5 m apart on flatter sections. The number of elevation
measurements per cross-section varied from 32 to 50. Elevation
measurements were repeated on all BRES cross-sections
post-construction in November 2019.
In addition to elevational profiles, 18 BRES cross-sections were
evaluated pre-restoration to characterize habitat type
(riffle/pool/run), bank erosion, bed substrate, depth, and soft
sediment depth. Soft sediment depth was measured by inserting a
steel rod into the substrate until it was met with resistance and
measuring that depth against a leveling rod. Soft sediment depth
measurements were taken at five equidistant points through the
wetted width of each cross-section. Transect characterization data
sheets were repeated post-construction in November 2019 for 12 BRES
cross-sections (060 through 390).
Cross-sectional area was calculated pre- and post-restoration using
the lower of the two edge-of-cross-section elevations or, when there
was one, the highest point inside of the lowest edge of
cross-section as the upper cross-section limit. This upper limit was
used instead of bankfull elevation because we were interested in the
impacts of restoration activities on floodplain reconnection during
overbank events for both flood wave attenuation and signals of
CH4 and CO2 delivery
to the channel. Soft sediment depth per cross-section was calculated
pre- and post-restoration as the sum of five measurements across
each cross-section (left edge of water, three equidistant points
through the channel, and right edge of water).
CO2 and CH4 fluxes
were measured using a floating chamber method. A portable greenhouse
gas analyzer (Los Gatos Research U-GGA-915, San Jose, CA, USA) was
attached to a cylindrical white chamber (23 cm diameter x 23 cm
height) suspended with a loose tether from a boom. The boom was
extended from the streambank (BREF), the upstream side of a foot
bridge (WUP), or the upstream side of a road bridge (BREF, WMD,
WDN). The BRES sampling site was ~250 river meters below the BREF
sampling site, while the WMD sampling site was ~950 river meters
below the WUP sampling site, and ~1400 river meters above the WDN
sampling site. The rate of change of gas concentrations inside the
suspended chamber during three, five-minute trials was used to
calculate flux (f):
f = dc/dt * h (1)
where dc/dt = change in gas concentration (mmol
m-3) within the chamber during the
deployment and h is the chamber height above the water surface (m).
Examination of plots of CH4 concentrations
for replicate chamber measurements confirmed consistent linearity
during trials, thus we assumed that chamber measurements were
capturing diffusive CH4 fluxes alone. In
three instances, because of instrument failures, daily mean flux
measurements were based on fewer than three sequential measurements.
