Benchland Pit expansion: effect on Bison Creek flows

How much of the Bison Creek drainage area the expanded sand and gravel pit cuts off, and how much mean annual flow, September mean flow and 7Q10 drop, calculated at three points from the hydrology report, the phasing plan and the gage record. At the cutthroat trout spawning reach the effect is many times the 1.8% the report gives at the gage.

Hydrology report Sep 14, 2026Bison Creek, 9.16 sq mi at the gagePit 42 ac existing + 65 ac proposedPrepared Nov 5, 2026

Summary

Share of drainage area

At the spawning reach the pit takes 11.8%, not 1.8%

The hydrology report gives the share only at the gage: 107 of 5,865 acres (Table 5-2, p. 24). Only 1.41 sq mi drains to the cutthroat trout spawning reach at Benchland Springs (habitat memo, p. 7). Counting the diversion ditch and berms, the share there is 20.9%.

Action: add P1 and P2 to Tables 5-2 and 5-3.

Low flow

7Q10 at the spawning reach drops 35.1%, mean annual flow 20.9%

The pit sits in Lake Bonneville gravel that feeds the springs at the toe of the bench (p. 27). Distributing low flow by surficial geology, with a factor of 3 for gravel, cuts the 7Q10 at P1 from 0.178 to 0.115 cfs. The report applies the same percentage to every flow statistic.

Action: confirm base flow with low-flow measurements at Benchland Springs before the report is revised.

Cut-off area

The diversion ditch and berms make the cut-off area 77% larger

Sheet C-102 shows diversion ditch DD-1 along the north and east pit edge routing 72 acres of upslope ground to the settling pond, plus 10 acres inside berms. All of it is pumped to Dry Hollow, outside the Bison Creek basin. The cut-off area is 189 acres, not 107.

Action: rerun Table 5-2 with the ditch’s contributing area for each phase.

Cut-off share and flow reduction by calculation point

Choose a phase and assumptions. The share and all three flow statistics update at once for the three points. Click a subbasin, the pit, the ditch, the settling pond or a calculation point to see the figures and where they come from.

Assumptions
Ditch and berms per Sheet C-102, low flow weighted by surficial geology.
Pit extent
Cut-off area
Low flow distributed by
3,0
100
0

P1

Cut-off share of drainage area0
Cut-off area0
MQ0
MLQ0

Drainage area and pit

Schematic map after Figure 3-2 of the report. The calculation uses the areas in Table 3-1 and Sheet C-102.

Basin divideStreamLake Bonneville gravel (bench)Pit through selected phaseLater phaseGround above diversion ditchDewatering line out of basinCalculation point

Figures and source

Click the map or choose a point.

Reduction by point

Percent reduction in each flow statistic at the three points under the selected assumptions.

Calculation steps

    All calculation points

    Flows before and after the expansion under the selected assumptions, compared with the share the report gives at the gage. Click a row to select the point.

    Sources and method

    Sources

    • Hydrology_Report_Benchland_Pit_Expansion.pdfTalus Ridge Water Resources, Inc., Sep 14, 2026. Subbasins (Table 3-1, p. 11), surficial geology (Table 3-3, p. 13), site drainage (p. 9), share and flow reduction (Tables 5-2 and 5-3, pp. 24–25), springs at the toe of the bench (p. 27)42 pages
    • C-102_Phasing_and_Drainage_Plan.pdfBenchland Sand & Gravel, LLC. Phase limits, diversion ditch DD-1, berms, settling pond and dewatering line to Dry Hollow1 sheet, 1" = 200'
    • BisonCreek_Gage_FlowStats_WY1996-2025.xlsxUSGS streamgage, Bison Creek at 1600 North: mean annual flow 6.2 cfs, September mean flow 1.85 cfs, 7Q10 0.72 cfs30 water years
    • StreamStats_Basin_Report_Bison_Creek.pdfUSGS StreamStats (Utah): basin delineation and drainage areas at P1–P36 pages
    • Aquatic_Habitat_Memo_Bison_Creek.pdfTalus Ridge Water Resources, Inc. Bonneville cutthroat trout spawning reach at Benchland Springs (p. 7)18 pages
    • UGS_Surficial_Geology_Excerpt.pdfUtah Geological Survey 1:24,000 surficial geologic map, excerpt. Check of Table 3-31 sheet

    Method

    1. Subbasins D1–D5 and their areas were taken from Table 3-1 and checked against the StreamStats delineation. Each calculation point gets the sum of the subbasins upstream.
    2. The cut-off area for each phase was measured on Sheet C-102: the pit, the ground upslope of diversion ditch DD-1 and the bermed area at the settling pond.
    3. Mean annual flow was transferred from the gage by drainage-area ratio: unit runoff × area.
    4. September mean flow and 7Q10 were weighted by surficial geology. Each map unit gets a relative base-flow factor, and the gage value is distributed over the weighted area. With the same factor for every unit the result equals the report’s drainage-area ratio.
    5. Reduction = share routed out of the basin × cut-off (weighted) area × unit runoff, plus net evaporation from open water for the part of the water that stays in the basin.
    6. Results were compared with Tables 5-2 and 5-3 of the report, which cover the gage only.

    Assumptions and limitations

    • The base-flow factors (gravel 3, alluvium and colluvium 1, bedrock 0.6, lake silt and clay 0.4) are a simplification that shows sensitivity. Replace them with measured low flows or a groundwater model before the numbers go into a permit application.
    • P1 and P2 are ungaged; their flows are transferred from the gage. Irrigation diversions and return flows are not in the record adjustments.
    • The report states that pit dewatering is pumped to Dry Hollow (p. 9), so 100% routed out of the basin is the starting point.
    • Net evaporation from open water: 3.0 ft/yr for mean annual flow and 0.25 in/day for the low-flow statistics (late summer). These values are assumptions.
    • Finding 2 rests on the base-flow factors and the net evaporation values. Both should be reviewed by a licensed hydrologist or PE before the low-flow results are used.
    • Drawdown outside the pit limits and groundwater lag are not modeled. Water-right questions for intercepted groundwater and pond evaporation belong with the Utah Division of Water Rights and are not evaluated here.
    • The expansion is permitted through a county conditional use permit. Extraction of sand, gravel and rock aggregate is excluded from the definition of a mining operation, and so from mined-land permitting by the Utah Division of Oil, Gas and Mining (Utah Code § 40-8-4). Bedrock other than basalt falls outside that exclusion.
    • The wording of Utah Code § 40-8-4 was checked against the Legislature’s bill PDFs, not the official code page. Counsel should confirm the current text before it is cited in the application.
    • Coordinates are given as on Sheet C-102: Utah Central zone of the 1983 system (SPCS 83), NAD83(2011), in US survey feet. Utah Code Title 57, Chapter 10, as amended by H.B. 512 (2026), effective May 6, 2026, adopts the 2022 State Plane Coordinate System for use within the state, but NGS has not yet adopted SPCS2022 and NATRF2022 as official: NAD 83 and NAVD 88 remain the official datums of the National Spatial Reference System, and the NGS SPCS2022 products are still in beta (NGS, read Nov 5, 2026). The coordinates have therefore not been converted.
    • Whether submittals made after May 6, 2026 must be in SPCS2022 should be confirmed by a Utah licensed land surveyor.
    • The map is schematic. The calculation uses the areas in Table 3-1 and Sheet C-102, not the map graphics.
    • The Bonneville cutthroat trout spawning reach at Benchland Springs is taken from the aquatic habitat memo. A fisheries biologist should confirm its location and use with the Utah Division of Wildlife Resources.