Hat Creek Restoration Project: Assessment of Geomorphic Change, 2015 to 2016

Andrew L. Nichols 1, Eric J. Holmes1 and Carson A. Jeffres 1

1 University of California, Davis, Center for Watershed Sciences

Introduction: Hat Creek is one of the most famous “spring-river” fly fishing destinations in North America. The lowest

5.5 kilometers of the creek are designated a “Wild Trout Area” (WTA) by the California Department of

Fish and Wildlife (CDFW) (Figure 1), resulting in management to insure natural reproduction of native

fishes (CADFG, 1999). However, since the late 1970’s, declining aquatic habitat quality and resulting

fishing opportunities throughout the Hat Creek WTA due to sedimentation issues have prompted

assessments of hydrogeomorphic process and conditions in lower Hat Creek (Kondolf et al., 1994; Cook

and Ellis, 1998; Cook, 2000), and their possible effects on populations of managed fishes.

Declining fishing conditions through the Hat Creek WTA are largely attributed to segment-scale

sedimentation problems and resultant loss of formerly dense beds of aquatic vegetation that provided

the dominant structural habitat for aquatic invertebrates and wild trout (CADFG, 1999). Additionally, it

is suggested that burrowing muskrats have degraded stream banks, resulting in channel widening and

the introduction of additional sediment loads to Hat Creek (CADFG, 1999). The combined losses of

aquatic vegetation, channel bed aggradation and channel widening have led to the development of wide

and shallow channel reaches with diminished aquatic vegetation cover relative to historical conditions.

These aquatic habitats are poorly suited for wild trout, and thus have prompted recent efforts to restore

aquatic habitat in select reaches within the WTA.

Through consultation with CDFW, California Trout (Cal Trout) initiated a “pilot” restoration project

within the Carbon Reach of the Hat Creek WTA in October 2015 (Figure 1). The focus of this pilot project

was the introduction of large woody debris (LWD) structures to help stabilize fine sediment, increase

spatial variability in flow velocities and depths, and also provide overhead cover to wild trout. Using

high-resolution velocity and topographic data collected prior to and following the installation of LWD

structures in Hat Creek, the UC Davis Center for Watershed Sciences evaluated hydraulic and

geomorphic changes to the Carbon Reach associated with the restoration activities.

Project Area: Hat Creek is a spring-fed tributary to the Pit River in Shasta County of northern California (Figure 1).

Streamflow in Hat Creek is primarily derived from groundwater sourced from the infiltration of rainfall

and snowmelt through porous volcanic rocks along the northern flank of Mount Lassen and throughout

the Hat Creek Valley (Rose and Davisson, 1996; Rose et al., 1996). Most of this spring water emanates

from several large and geographically discrete springs. Volumetrically, the largest springs in the Hat

Creek Valley are Rising River and Crystal Lake, which combined contribute as much as 400 ft3/s of

streamflow to Hat Creek. Temporal variation in spring-flow magnitudes is tightly coupled to regional

precipitation magnitudes (Rose et al., 1996; Manga, 1999). Groundwater emanating from the large

springs in the Hat Creek Valley are cool (7-8 °C), nutrient rich (Lusardi, 2014; Lusardi et al., 2016) and

exhibit water quality characteristics suggestive of interactions with regional sources of geothermal heat

and carbon (Rose and Davisson, 1996; Rose et al., 1996).

Figure 1: Hat Creek Wild Trout Area

Streamflow in Hat Creek is diverted through two hydroelectric facilities, commonly referred to as the

Hat Creek #1 and Hat Creek #2 Powerhouses. The Hat Creek WTA extends approximately 5.5 km from

below the Hat Creek #2 Powerhouse to Lake Britton (Figure 1). Similar to most spring-fed creeks located

throughout the Cascades of northern California and southern Oregon (Whiting and Stamm, 1995;

Whiting and Moog, 2001), streamflow magnitudes through the WTA are quite stable. Daily streamflow

during water years 2007 through 2015 was estimated by summing flows diverted through the Hat Creek

#2 Powerhouse (USGS Site ID 113593) and measured in the Hat Creek bypass reach (USGS Site ID

113592). During this period, mean daily flows through the WTA were 388 ft3/s (σ = 75 ft3/s). Historical

measurements within the WTA (USGS Site ID 113593; 1921-1922) identified stable flows averaging

approximately 490 ft3/s.

The Hat Creek WTA exhibits low stream gradients and rectangular channel morphologies consistent with

many volcanic spring-fed rivers (Whiting and Moog, 2001). Channel widths throughout the WTA

average approximately 38 m, and channel depths generally range from 1-2 m (Kondolf et al., 1994).

Woody riparian vegetation along the banks of the WTA is limited to a small number of alders, while

many of the streambanks and near-shore areas are occupied by emergent aquatic vegetation. Due to

unshaded, low-gradient, wide and shallow channel conditions, submerged aquatic vegetation

historically proliferated, providing the dominant structural habitat for trout.

Sedimentation: Volcanic spring-fed rivers typically occupy landscapes devoid of surface water networks whose flows and

ability to deliver sediment through tributary networks rise and fall in concert with local precipitation.

Consequently, sediment supply to volcanic spring-fed rivers is generally limited, and typically only occurs

following bank erosion and other episodic and low-frequency sediment delivery events. Furthermore,

stable streamflows, low stream gradients and the absence of large floods generally precludes the

downstream transport of bedload sediments larger than sand (<2 mm). Such hydrogeomorphic

conditions generally lead to stable channel morphologies whose variability in forms are often dictated

by ecological engineers such as aquatic macrophytes, large woody debris (LWD) and even riverine fauna

(beavers, muskrats, spawning fish).

Historically, the Hat Creek WTA likely exhibited the stable morphologies characteristic of many volcanic

spring-fed rivers. These conditions allowed aquatic macrophyte communities to flourish, helping to

provide abundant, high quality aquatic habitat for fish. However, by the late-1970’s, fisherman and

other observers identified an apparent increase in fine sediment throughout the WTA, which was

helping to aggrade the river channel and bury the existing aquatic vegetation. Several studies

performed to evaluate the source, fate and transport of this sediment (Kondolf et al., 1994; Cook and

Ellis, 1998; Cook, 2000) suggest sinkholes in Lake Baum facilitated the subterranean transport of as

much as 64,000 cubic yards of sand-sized sediments to discrete spring sources along the Hat Creek #2

bypass reach (Figure 1). Following these sediment delivery events in the 1970’s, bypass flows

transported the sediments into the WTA, where stable spring-flows promoted the slow, downstream

migration of a sediment wave (sensu Madej and Ozaki, 1996). By the mid 1990’s this sediment wave

had passed through the Carbon Reach of the WTA, and is anticipated to exit the WTA by 2030 (Cook and

Ellis, 1998; Cook, 2000). During the decades-long downstream transport of the sediment wave, large

volumes of fine sediment have been stored within elongated lateral bars in over widened reaches of the

WTA, such as observed within the Carbon Reach (River Run Consulting, 2014). Documenting the fate of

this large sediment source, determining its effect on aquatic habitat, and ultimately managing the

sediment source have become the defining scientific and conservation issues facing the Hat Creek WTA.

The Carbon Reach One of the widest and shallowest reaches throughout the WTA is known as the “Carbon Reach”, located

approximately one kilometer downstream from the Hat Creek Powerhouse #2 (Figure 1). The historical

location of the Carbon Bridge and a popular fishing location, this straight, 400-meter reach is

anomalously wide, with a pronounced depositional bar creating a wide and shallow area on the left

(south) side of the river. Bank erosion issues associated with cattle grazing, muskrat burrowing and

excessive fishing pressure led to the stabilization of the right bank of the Carbon Reach in early 2000’s.

However, muskrat burrowing continues to facilitate the erosion of banks throughout the reach (River

Run Consulting, 2014).

To facilitate improvement of aquatic habitat throughout the Carbon Reach, Cal Trout recently initiated a

pilot restoration project using engineered large woody debris (LWD) structures. Assessment of

naturally-recruited large woody debris (River Run Consulting, 2014) identified highly variable

topographic and flow velocity conditions within and around existing logs in Hat Creek. The placement of

engineered wood structures along the left bank of the Carbon Reach aimed to replicate these

hydrogeomorphic conditions, helping to create more complex aquatic habit for trout. In November

2015, four large woody debris structures (herein identified as LWD_1 through LWD_4) were placed

along the left bank of the Carbon Reach within the Hat Creek WTA (Figures 2, 3). The structures were

lowered into the river by crane and helicopter, and were subsequently secured to the streambank.

Figure 2: Four (4) large woody debris [LWD_1 (upstream) through LWD_4(downstream)] structures were placed

along the left bank of the Carbon Reach in November 2015 (image from September 2016).

The steepness of the terrace-type stream bank (see River Run Consulting, 2014) on river left of the

Carbon Reach necessitated the LWD structures be ballasted well above the water surface of the

adjacent creek (Figure 3a). With the length of the longest wood elements exceeding 30 meters, the

elements gradually transition from being suspended above the water surface to lying on the bed of the

creek (Figure 3b). The near bank (southern) portions of each LWD element rest at or above the water

surface during baseflow conditions and do not affect channel hydraulics. As the elements extend

northward and out into the creek, they gradually become more submerged until they are fully

submerged and lying on the channel bed. Partially and fully submerged portions of the LWD elements

closest to the left bank of the creek allow streamflow to pass underneath. The northern portions of

these submerged wood elements rest on the channel bed and largely prevent streamflow from passing

underneath.

Figure 3: LWD structures placed along the left bank of the Carbon Reach include wood elements with segments

that are above the water surface, partially submerged and suspended above the channel bed, and fully

submerged and lying on the channel bed. A = LWD_2; B = LWD_3.

Restoration Assessment Methods To quantify geomorphic and hydraulic effects of the placement of large wood debris along the left bank

of the Carbon Reach, UC Davis Center for Watershed personnel performed detailed surveys of flow

velocities and channel bed topography throughout the reach prior to (September 2015) and following

(September 2016) LWD structure placement.

Flow Velocities During the pre- and post-restoration monitoring periods, flow velocities were measured at 1 meter

intervals across 16 channel cross sections (Figure 4) placed within and between the four LWD structures.

Flow velocity measurements were collected September 2-3, 2015, and September 6-7, 2016 during

periods of similar baseflow magnitudes. Measured stream flow on September 3, 2015 was 333 ft3/s,

while streamflow on September 7, 2016 was 336 ft3/s. All velocity measurements were collected at 0.6

of the measured water depth and represented average flow velocities within the water column.

Measurement locations from the 2015 sampling period were surveyed with a realtime kinematic (RTK)

global positioning system (GPS) survey unit (measurement accuracy and precision ± 0.02 m), allowing re-

occupation of the same measurement in points in 2016.

To assess changes in flow velocities prior to (2015) and following (2016) placement of the LWD

structures, the difference in flow velocity at each measurement location was calculated. From these

measurement differences, an inverse distance weighted (IDW) interpolation was performed in the

Geographic Information System (GIS) ArcMap 10.3 to generate a 1m x 1m pixel resolution raster of

velocity changes between September 2015 and September 2016.

Figure 4: Channel cross section and velocity profile locations throughout the Carbon Reach of Hat Creek. A

velocity profile at cross section 17 was not conducted as water depths precluded use of the measurement

equipment.

Channel Bed Topography Prior to and following LWD placement, high-precision channel bed topographic surveys were completed

using multiple RTK GPS survey units. All topographic data were georeferenced to the National Geodetic

Survey (NGS) benchmark AF8156 (Table 1) located along CA State Highway 299

(https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=AF8156), and were collected within the NAD 83

(2011) horizontal and NAVD88 vertical datums, using a UTM Zone 10N projection. Using positional data

from NGS benchmark AF8156, two local topographic benchmarks (BM_1, BM_2) (Table 1) were

established along the right bank of the Carbon Reach using RTK GPS. Both benchmarks were

monumented through the placement of 3/8” rebar within 2-ft deep concrete footings. 30-second data

sampling intervals enabled the collection of high precision (± 0.003 m) horizontal and vertical

measurements at each benchmark location, from which all subsequent topographic data were

georeferenced.

Name Northing (m) Easting (m) Elevation (m)

AF8156 4537869.387 622574.201 851.5

BM_1 4536341.778 622117.001 847.481

BM_2 4536411.313 622031.317 853.437

Table 1: Hat Creek topographic benchmark locations [UTM Zone 10N projection, NAD 83(2011) datum]

Pre and post-restoration topographic surveys were completed throughout an approximately 250 m long

portion of the Carbon Reach bounded by upstream (XS 1) and downstream (XS 17) cross-sectional

survey locations (Figure 4). The surveyed reach had an areal extent of 10,047 m2. During September 2-

5, 2015, 7,432 individual topographic data points were collected throughout the surveyed reach (0.73

points/m2 surveyed) prior to the placement of the LWD structures. Post-restoration topographic

surveys were completed throughout the same area between September 5 and 8, 2016. During the 2016

survey, 11,658 topographic data points were collected (1.16 points/m2 surveyed).

Digital Elevation Models (DEM’s) of channel bed topography prior to and following LWD placement were

developed in ArcMap 10.3.1 following methods detailed in Wheaton, et al. (2010). Pixel resolution for

each spatially concurrent DEM was 0.5 m, enabling detailed comparison of channel topography changes

in reach locations with expected geomorphic change and correspondingly high survey point density (i.e.

in and around LWD structures). Differencing the concurrent DEMs created from the 2015 and 2016

topographic data sets facilitated the creation of a DEM of difference (DoD), enabling detailed analysis of

sediment deposition and erosion patterns throughout the surveyed reach following LWD placement.

To quantify changes in channel bed topography following the placement of the LWD structures, we

calculated both the net change in sediment storage, as well as changes in terrain roughness (i.e.

topographic variability) across the surveyed area. To calculate the change in sediment storage, we

subtracted the volume of eroded bed sediments from the volume of deposited bed sediments using the

DoD. To quantify changes in topographic complexity, we used focal statistics in ArcMap 10.3 to

calculate the standard deviation of elevation for each 3x3 pixel square neighborhood (1.5 x 1.5 m) in the

surveyed reach in both 2015 and 2016 (Scown et al., 2015). Differencing the concurrent rasters of the

standard deviation of elevation from the 2015 and 2016 sampling periods enabled the geospatial

patterns of either increasing (+ standard deviation) or decreasing (- standard deviation) topographic

variability throughout the entire surveyed reach.

Results

Flow Velocities Prior to the installation of the LWD structures, geospatial patterns of flow velocities reflected the long-

term development of an over-widened channel with a shallow bar of the left side of the river and a deep

run along river right (River Run Consulting, 2014). Flow velocities were low throughout the extent of the

shallow bar, particularly in regions with extensive growth of submerged aquatic macrophytes (Figure

5a). Within macrophyte patches, flow velocities were homogenously low, and often negative in eddies

behind patches. The deeper, river right channel run was largely devoid of macrophyte growth and

exhibited much higher flow velocities (0.3 – 0.6 m/s) (Figure 5a). Edge roughness associated with bank

friction and emergent vegetation reduced flow velocities on both sides of the Carbon Reach

Following placement of the LWD structures, general flow velocity patterns throughout the surveyed

reach remained largely unchanged. Lower velocities were observed along the shallow and vegetated

left side of the creek, while higher velocities were observed along the deeper and less vegetated right

side of the creek (Figure 5b). However, velocity patterns within the LWD structures exhibited greater

variability relative to the pre-restoration survey period. Generally, flow velocities decreased behind

partially and fully submerged portions of the LWD elements, while increasing in areas where the

elements remained above the water surface (Figure 6). The greatest changes in flow velocity (both

positive and negative) were observed within the two upstream LWD structures (LWD_1 and LWD_2).

The downstream (LWD_4) structure was a notable exception to this pattern. Within this structure, flow

velocities uniformly decreased (albeit slightly) following LWD placement.

Figure 5: A) Flow velocities through the Carbon Reach prior to LWD placement (2015); B) Flow velocities post-

LWD placement (2016).

Figure 6: Changes to flow velocities throughout the Carbon Reach between September 2015 and September

2016. A) Differences in flow velocities at each transect measurement point (2016 – 2015); B) Rasterized data of

flow velocity changes within and between each LWD structure. Reds = velocity increase; Blues = velocity

decrease (measurement error = ± 0.02 m/s).

Channel Morphology Channel bed morphology throughout the Carbon Reach was remarkably simple prior to the placement

of LWD, principally characterized by a deep run on river right and a shallow lateral bar on river left

(Figure 7). However, following placement of the LWD structures, subsequent changes to hydraulic

conditions (i.e. flow velocities) initiated considerable geomorphic change illustrated by geospatial

patterns of channel bed erosion and deposition (Figures 8, 9a). Notable differences in the topography of

the lateral bar on river left are readily apparent in the vicinity LWD structures 1, 2 and 3 (Figure 8b),

where topographic conditions in 2016 appear much more variable relative to the 2015 period.

Figure 7: Pre-restoration (2015) channel bed topography throughout the Carbon Reach. A) Digital

orthophotograph created from images collected in September 2015. B) Digital Elevation Model (DEM) of the

Carbon Reach in September 2015. DEM pixel size = 0.5 m.

Changes in topography following LWD placement were quantified through the generation and analysis

of a DEM of Difference (DoD) for the 2015 and 2016 monitoring periods. Generally, the DoD identified

pronounced and localized channel bed erosion underneath partially and fully submerged portions of the

LWD structures (Figure 9a). Within the upstream structures (LWD_1 and LWD_2), bed erosion in excess

of 0.45 m was observed. Broad zones of sediment deposition were observed downstream from the

locations of channel bed scour, identifying locations where the scoured sediment was deposited.

Minimal geomorphic change was observed upstream from any of the LWD structures. Consistent with

observed geospatial variation in the magnitude of flow velocity differences between 2015 and 2016, the

greatest geomorphic changes were located within the upstream-most LWD structures (LWD_1 and

LWD_2).

Figure 8: Post-restoration (2016) channel bed topography throughout the Carbon Reach. A) Digital

orthophotograph created from images collected in September 2016. Note placement of (4) LWD structures and

new Carbon Bridge. B) Digital Elevation Model (DEM) of the Carbon Reach in September 2016. DEM pixel size =

0.5 m.

Channel bed erosion and deposition throughout the left side of the Carbon Reach following LWD

structure placement also resulted in increased topographic complexity. Differences in the standard

deviation of channel bed elevation prior to (2015) and following (2016) LWD placement identified

prominent zones of increased topographic complexity (Figure 9b). The greatest increases in topographic

complexity were co-located with the areas surrounding submerged LWD elements, where the greatest

amount of channel bed scour or deposition was observed. Very few locations within the Carbon Reach

exhibited reduced topographic complexity following LWD structure placement.

Figure 9: Post-restoration changes to channel bed topography throughout the Carbon Reach. A) Changes in

channel bed elevation (± m) between 2015 (pre-restoration) and 2016 (post-restoration). Reds = sediment

deposition; Blues = sediment erosion. B) Change in the standard deviation of channel bed elevation (± m).

Oranges/Reds = increase in topographic complexity; Greens = decrease in topographic complexity.

The DEM of Difference (DoD) (Figure 9a) was also used to quantify the reach-scale change in sediment

storage following placement of the LWD structures. The majority of channel bed erosion and deposition

occurred within the river left lateral bar on which the LWD structures were placed. Throughout the

entire surveyed reach, there was a net increase in sediment storage (+ 4.27 m3) (Figure 10), with much

of the storage occurring within the lateral bar and behind the LWD structures (Figure 9a).

Figure 10: Histogram of channel bed elevation change throughout the Carbon Reach between September 2015

and September 2016.

Discussion In spring-fed rivers with stable flow regimes, large wood can remain remarkably stable over time, and is

generally a dominant control on channel hydraulics when present (Manga and Kirchner, 2000). The

stable flow regimes and limited sediment supplies typical to many spring-fed rivers generally result in

the persistence of homogenous channel forms consisting of shallow, wide and rectangular channel

morphologies (Whiting and Moog, 2001; Reiser et al., 2004). Additionally, alluvial bars and other

geomorphic features common to river systems that exhibit a wider range of flow magnitudes are largely

absent in spring-fed rivers (Whiting and Moog, 2001). With such intrinsically homogenous channel

morphologies, hydraulic conditions and flow regimes, large woody debris and other biotic structures

(e.g. aquatic macrophytes) can play a large role in generating geomorphic and habitat complexity in

spring-fed rivers (Manga and Kirchner, 2000).

Wood principally alters channel hydraulics and morphology by obstructing flow and sediment transport,

with implications across multiple spatial and temporal scales (Montgomery et al., 2003). At the local

scale (meters to 10’s of meters), wood-mediated changes to flow velocities and bed shear stresses

facilitate geomorphic responses such as the scour of pools and creation of channel bars. Such

geomorphic responses are largely dependent on the size, shape, porosity and orientation of the wood

structures (Montgomery et al., 2003; Manners et al., 2007). For example, horizontal logs located above

the channel bed force water to accelerate under the log and towards the channel bed. If the underlying

bed materials are mobile, this promotes pool scour under and behind the logs (Cherry and Beschta,

1989; Wood-Smith and Buffington, 1996). Manners, et al. (2007) show that the porosity of a wood

obstruction can strongly affect spatial patterns of flow velocities and bed shear stresses, with

implications for resulting patterns of sediment scour and deposition which subsequently determine

aquatic habitat complexity.

The complex construction of wood structures throughout the Carbon Reach of Hat Creek facilitated local

variability in the hydraulic and geomorphic response to the restoration action. Where wood pieces

were oriented orthogonal (north to south) to the dominant direction of flow (east to west),

hydrogeomorphic responses were principally dictated by the location of the placed log relative to both

the water surface and the channel bed. Locations where the logs were placed within the water column

but suspended above the bed of the channel exhibited the greatest channel bed erosion (Figures 9a, 11).

This suggests that flow acceleration beneath the logs and towards the channel bed resulted in

underscour of the highly mobile sand-size materials on the depositional bar. The magnitude of

underscour decreased in locations where LWD elements rested on or only slightly above the channel

bed. Downstream from these zones of underscour, flow velocities decreased dramatically due to both

the blockage of flow and eddies created from the detachment of flow (sensu Manners et al., 2007) as it

was deflected around the LWD structure. This reduction of flow velocity within and behind the

structures resulted in the expansive deposition of sediments (Figures 9a, 11), largely sourced from the

upstream underscour.

Figure 11: Conceptual depiction of flow acceleration and underscour under horizontal LWD pieces, and flow

detachment and eddy deposition of sediment behind LWD obstructions.

The hydrogeomorphic effects of LWD placement were greatest within and around the upstream

structures (LWD_1 and LWD_2). Changes in flow velocities and channel bed topography were greatly

diminished in the downstream-most structure (LWD_4), suggesting that the upstream structures were

“shielding” LWD_4 from geomorphic change by progressively reducing flow velocities and shear stress

on the downstream portion of the depositional bar. Reductions of flow velocities throughout the

downstream portions of the bar were also likely influenced by the slight downstream orientation of the

log structures. The orientation of these logs helped to deflect higher velocity flows capable of channel

bed erosion to the north and into the deeper parts of the channel.

Ultimately, two principal effects of LWD placement throughout the Carbon Reach of Hat Creek were

observed within the first year following placement of the LWD. First, by obstructing flow and altering

channel hydraulics, the LWD structures induced highly localized changes to flow velocities and channel

bed topography, with likely benefits to aquatic biota reliant on river channel complexity (Wohl, 2016).

Second, the net deposition (+ 4.27 m3) of sediment throughout the surveyed reach indicates that the

placement of LWD structures helped to aggrade to the depositional bar on river left. This can be

considered the first step in the narrowing of the channel throughout the reach. It is likely that this

pattern of channel aggradation of the river left depositional bar will continue over time.

The long-term trajectory of hydraulic and geomorphic conditions throughout the Carbon Reach will

largely depend on the evolution of the LWD structures. The initial structures were built through the

placement and anchoring of individual logs, often referred to as “key members” of debris jams (Manners

et al., 2007). However, naturally recruited debris jams are generally formed through the aggregation of

wood materials ranging in both size and density. The combination of key members and an evolving

matrix of wood materials typically decreases debris jam porosity and increases local variability in

channel hydraulics and patterns of sediment deposition and erosion (Manners et al., 2007). Through the

Carbon Reach, the continued trapping of woody materials or the possible recruitment and growth of

emergent vegetation in aggrading low velocity areas within and behind the wood structures is likely to

alter local hydraulics within each jam over time, leading to further geomorphic change. However, given

the generally stable hydrologic regime of Hat Creek and the trapping of upstream sources of wood in

Lake Baum, this process of wood recruitment and debris jam evolution is likely to be slow. Infrequent,

large floods may also promote local changes in channel bed topography as flood flows in excess of the

commonly observed spring-fed baseflows interact with the LWD structures.

Conclusions Data collected during this assessment of initial hydraulic and geomorphic responses to the placement of

four (4) large woody debris (LWD) structures along the left bank of the Carbon Reach of Hat Creek

support the following conclusions:

1) By blocking and re-routing flow pathways, the LWD structures promoted local variability in

hydraulic conditions, resulting in significant local changes to channel bed topography within one

year of LWD placement.

2) Geomorphic change was principally characterized by underscour beneath the upstream log of

each LWD structure and eddy deposition downstream from this log. Underscour magnitudes

were greatest beneath submerged LWD elements suspended above the channel bed. Bed

materials scoured from under these logs were deposited immediately downstream.

3) Hydrogeomorphic change was greatest in the vicinity of upstream LWD structures. It is likely

that these structures “shielded” the downstream structures by deflecting flow towards the

deeper portions of the river right channel, thus minimizing the interaction of higher velocity

streamflows with the downstream structures needed to initiate geomorphic change.

4) The Carbon reach exhibited net sediment deposition over the year following LWD structure

placement. The majority of this sediment was deposited behind the LWD structures, helping to

aggrade the river left sediment bar.

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