Jens Ammon
Riverscape Diversity within the Blacksmith Fork River
Blacksmith Fork Canyon
Utah
The Blacksmith Fork River is the trunk stream within the Blacksmith Fork Watershed, a ~743 square kilometer drainage area in the Bear River Mountains. This mountain range formed as part of the Basin and Range extension, with much of the bedrock drained by the Blacksmith Fork River being comprised of Middle Cambrian (541-485 Ma) limestone. The mainstem length of the Blacksmith Fork River is ~45.9 km, with an elevation change of ~605 m out of the ~1465 m total relief of the watershed.
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The Blacksmith Fork Watershed is triangular in shape, with a circularity ratio of 0.33 and an elongation ratio of 0.85. While the circularity ratio indicates the watershed is elongate, the elongation ratio indicates it is more round. This discrepancy is likely due to the two spikes coming off the watershed at the north and west sides of the watershed throwing off the circularity ratio.
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Watershed Summary
Blacksmith Fork Watershed
Area: 743 sq. km
Relief: 1465 m
Circularity Ratio: 0.33
Elongation Ratio: 0.85
Blacksmith Fork River
Length: 45.9 km
Relief: 605 m
The Blacksmith Fork River is a snowmelt dominated system, with peak flows in the spring as the snowpack begins to melt. High elevation snowpack then supplies water into the summer. The right graph shows the Blacksmith Fork discharge in cfs for the 2019-2020 water year.
Since most of the watershed lies within the Bear River Mountains, the dominant class of vegetation cover is open and closed tree canopy, with large areas of sagebrush-steppe.
Nearly all of the watershed is classified as a Northwestern Forested Mountain ecoregion, with only the lowest tip falling under the North American Desert ecoregion. As such, a separate ecoregion map was not included.
Longitudinal profile of the Blacksmith Fork River. The river has an elevation change of ~605 m over a length of ~45.9 m. A small knickpoint exists near the middle of the river due to a small hydroelectric dam.
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To compare the riverscape diversity within the Blacksmith Fork system, three reaches of varying confinement were chosen along the length of the river. The locations of these reaches are indicated on both the longitudinal profile of the river and the below map. In the following sections, each of these reaches will first be analyzed individually, then all three will be analyzed together in the context of the overall watershed.
Reach 1
Reach 1 Summary
Channel Length: 1732 m
Valley Length: 1267 m
Sinuosity: 1.4 (Sinuous)
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Average Width: 7-9 m
Average Flow Speed: ~0.5 m/s
Bed Material: Fine
Active Channels: 1-2
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Confinement: 23% (Partly-Confined)
Reach 1 is a partly confined, sinuous, single-channel riverscape just downstream of the confluence of Blacksmith Fork River and Mill Creek. The channel generally has a low slope, but there are occasional large bedrock forced waterfalls up to ~5 m in height.
There are frequent deposits of fine silt along the banks, with coarser deposits of cobbles up to ~10 cm in the thalweg, but these larger clasts often have fine silt sediment between them. Occasional bedrock outcrops are visible on the channel bottom. Woody vegetation is sparse to nonexistent, with only the occasional large shrub tree. However, grasses are prevalent across the floodplain, causing high cohesion of the banks.
(Above) The channel of reach one was ~7-9 m wide, with a single thalweg ranging from ~0.5-1.5 m in depth. The thalweg is often not distinct.
Reach 1 is dominated by planar geomorphic units such as glides and runs. There are occasional step pools and cascades due to small bedrock outcroppings. There are also occasional large (~3-5 m) bedrock waterfalls and plunge pools. Small point bars are common on the inside bank of channel bends, and occasional expansion bars are present.
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Backwaters and wetlands are common over the floodplain, with channel boundaries being poorly defined. The floodplain builds primarily through lateral accretion, however, sheet deposits would occur during high flow events due to the low banks in the wetland areas.
Reach 2
Reach 2 Summary
Channel Length: 887 m
Valley Length: 410 m
Sinuosity: 2.2 (Tortuous)
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Average Width: 6-8 m
Average Flow Speed: ~0.75 m/s
Bed Material: Gravel
Active Channels: 1-3
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Confinement: 0% (Unconfined)
Reach 2 is a laterally unconfined, tortuous, multi-channel riverscape just upstream of the confluence of Blacksmith Fork River and Rock Creek. The channel and the surrounding valley have a low slope.
There are frequent deposits of fine silt along the margins of the channel and in eddy pools, but the channel bed is dominated by gravel and cobbles ranging up to ~10 cm. Occasional bedrock outcrops are visible on the channel bottom. Large trees are sparse, but woody shrubs are common and grasses are prevalent, causing high cohesion of the banks.
(Above) The channel of Reach 2 was ~6-8 m wide and generally shallow (~0.1-0.5 m) One dominant thalweg moves within the channel to hug the outside bank at bends, but secondary thalwegs were often present. Deep pools were common along the main thalweg.
The geomorphic unit assemblage of Reach 2 is the most diverse of the three studied reaches. Structurally forced pools are common where roots or other debris intersect the thalweg. Large point bars are common at the inside bank of bends. Longitudinal bars, diagonal bars, and riffles are also common within the channel.
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Lateral connectivity within this reach is high. There are frequent backwaters and backswamps leading to wetland/marsh areas around the main channel. During one section of dense vegetation, the main channel splits into multiple small channels similar to what might happen as a result of beaver dam activity. Chute cutoffs and neck cutoff channels are common on the larger meander bends. The floodplain builds primarily through lateral accretion, as indicated by the strong point bars and large sections of bank erosion (see picture).
Reach 3
Reach 3 Summary
Channel Length: 1019 m
Valley Length: 943 m
Sinuosity: 1.1 (Low Sinuosity)
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Average Width: 6-8 m
Average Flow Speed: ~1 m/s
Bed Material: Boulder
Active Channels: 1
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Confinement: 91% (Confined)
Reach 3 is a confined, single-channel riverscape of low sinuosity at the mouth of Blacksmith Fork Canyon. Portions of the reach have a high slope and cascading flow, but other portions of the reach have a moderate to low slope and are dominated by rapids or runs.
While there is the rare fine deposit at along at a sheltered margin, the channel is dominated by coarse cobbles and boulders ranging in size from 15 cm to 2+ m. Talus slope margins are common, supplying the coarse channel sediment. Despite the very limited channel margin space, vegetation is prevalent, including some larger woody plants with trunk diameters up to ~6 inches. The grasses that were seen at the two upstream reaches are not prevalent here.
(Above) The channel of Reach 3 was ~6-8 m wide with a deep 1-2 m thalweg. Channel shape is largely determined by the size of the boulders present in the channel. The thalweg in areas with many large boulders is generally less well-defined than areas with smaller bed material.
Reach 3 is dominated by rapids and cascades. Sections of the reach with more large boulders fallen in from the confining talus slopes and hillslopes are dominated by cascades with frequent eddy pools, plunge pools and hydraulic jumps. Sections with less large boulder debris input are dominated by runs and rapids, with forced pools due to woody material and the occasional large boulder.
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The floodplain in this reach is made up of discontinuous pockets. While the floodplain on river-right may have been larger historically, Blacksmith Fork Canyon Road has greatly reduced the possible valley bottom. Floodplain geomorphic units were not present.
Reach Intercomparison
Unlike textbook examples of a confined riverscape gradually transitioning to an unconfined system, the Blacksmith Fork River transitions back and forth multiple times along the length of the watershed. The main factor contributing to this variation in confinement is the surrounding geology and hillslopes. Valley widths start out narrow at the headwaters, but then widen significantly around Reach 1 and 2. The hillslopes at the top of the mountain range are also shorter and less rocky than those near the lower portions of the canyon. This valley configuration allows the river to expand laterally, with sediment being supplied by lateral erosion and small alluvial fans.
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Once the river moves into the lower parts of Blacksmith Fork Canyon, the confining hillslopes are much steeper, much taller, have more rocky outcrops, and the valley width is much narrower. This valley configuration force the riverscape into a partly-confined to confined setting, with the UT-101 highway adding to the degree of confinement. Sediment sources along these reaches are primarily from talus slopes and rockfalls from the surrounding hillslopes, leading to much more frequent cascades and turbulent features than the upstream reaches.
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Roads and Anthropogenic development also limit the lateral connectivity and sediment supply of the river, especially in the lower canyon were the valley is already narrow. Within the canyon, sediment from the hillslopes on the other side of the road is disconnected from the channel and valley floor. While large landslides may move enough sediment to pass over the road barrier, the riverscape generally does not receive input from that side of the valley. Beyond the canyon, agriculture and residential development prevents lateral migration of the channel. While the slope is low and the river is sinuous in this area, conditions that might otherwise suggest high lateral connectivity, a comparison of historical aerial photographs shows very little lateral movement of the channel.
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Despite the constraints imposed by anthropogenic development, some anthropogenic land management is helping the riverscape diversity of the canyon. Large sections of river in the upper portions of the canyon is being restored and maintained by Hardware Ranch WMA and Utah DWR. Reach 2 would likely be much more confined if not for these groups. Also, beaver dam analogs can be seen in many of the Blacksmith Fork River tributaries.
Hillslope margins of the upper canyon (left) and the lower canyon (right). Shallow, rounded hillslopes and a wide valley in the upper canyon allow the riverscape to less laterally confined than the steep, rocky hillslopes and narrow valley of the lower canyon.