Numerous 3rd-order mountain catchments within the Cache la Poudre (Poudre) River basin in the Colorado Front Range, USA burned severely and extensively during the 2020 Cameron Peak fire. Many of these catchments experienced debris flows and flash floods triggered by convective storms after the fire. The downstream effects of the debris flow sediment varied along a continuum from attenuated and largely contained within the catchment, through contributing to a pre-existing debris fan at the catchment outlet, to releasing substantial volumes of water and sediment to the Poudre River. We conducted longitudinally continuous surveys within seven of these catchments to measure reach-scale characteristics. These characteristics quantify the reach-scale geomorphic, vegetation, and burn characteristics, with a particular focus on elements that introduce inter- and intra-reach spatial heterogeneity including channel planform, beaver modified topography, the distribution of channel and floodplain logjam distribution density, and the floodplain width/channel width ratio for the population of reaches within each catchment.

We conducted a longitudinal reach survey within each study catchment. The survey started at the drainage divide, moving downhill to find the first evidence of channelization and follow it to the stream head. We followed the river downstream, defining the boundaries between reaches by field-observed relative valley and channel geometry using a Garmin GPSMAP 66st and a Garmin eTrex10 with 3-m horizontal accuracy. Within each reach I documented burn status, vegetation type, basal area, channel planform, channel bedform, bankfull width, floodplain width, number of channel and floodplain jams, and number of beaver berms. Burn status was documented categorically in the field as burned, unburned, or mixed. Vegetation type was noted categorically as conifer, willow, aspen, or herbaceous based on the species present, where herbaceous denoted a lack of trees rather than implying that there was no herbaceous vegetation where other species were present. Basal area of the river corridor forest was measured with a Panama Angle Gauge at approximately one channel width away from bankfull. All standing trees (living and dead) were included. Channel planform and channel bedform were determined categorically. Bankfull channel and floodplain widths were measured using a Laser Technology TruPulse 360B laser range finder and high-water marks indicative of high flows from the most recent spring snowmelt flows. Logjams were designated as at least three pieces of wood in contact, with each piece of wood at least 1 m in length and 10 cm in diameter. Jams within the bankfull channel were tallied as channel jams and jams outside of the bankfull channel but within the bounds of high-water marks were tallied as floodplain jams. The presence of beaver berms was tallied within each reach where clearly defined individual berms existed. Where there were complex beaver meadows, beaver modified terrain was noted as “beaver meadow,” indicating many berms that were not distinct throughout the length of the reach.