Pre-analysis per-gauge facts (today, D76): - Boxley: first 2.10 ft @ 00:00; max 2.11 ft @ 00:30; last 2.03 ft @ 23:30. Pure recession with one-step within-day micro-rise. - Ponca: first 161 cfs @ 00:00; max 161 cfs @ 00:00 (early); last 87.7 cfs @ 23:30. Step-drop at 11:00 from 154 → 91 cfs (rating-curve shift, not hydrologic). - Pruitt: first 3.72 ft / 108 cfs @ 00:00; height max 3.74 ft @ 00:30 / discharge max 113 cfs @ 00:30; last 3.66 ft / 96.2 cfs @ 23:30. - St. Joe: first 3.88 ft / 321 cfs @ 00:00; height max 3.89 ft @ 01:15 / discharge max 324 cfs @ 01:15; last 3.75 ft / 279 cfs @ 22:45. - Harriet: first 4.02 ft / 414 cfs @ 00:00; height max 4.02 ft @ 00:00 / discharge max 414 cfs @ 00:00; last 3.92 ft / 358 cfs @ 23:45. - Richland: first 1.51 ft @ 00:00; max 1.52 ft @ 00:15; last 1.41 ft @ 23:00. - Bear Creek: first 2.16 ft / 17.0 cfs @ 00:00; max 2.16 ft @ 00:00 / discharge max 17.0 cfs @ 00:00; last 2.12 ft / 15.0 cfs @ 23:15.
Light, scattered convective shower activity in the afternoon/evening — basin-max 24-hr 0.065" in Ponca zone. Spatially concentrated in the Ponca/Boxley headwaters: HUC12 0202 (Beech Cr) 0.072"/0.049" 1-hr, 0203 (Smith Cr) 0.062"/0.050" 1-hr, 0201 (Boxley) 0.054"/0.032" 1-hr, 0205 (Whiteley) 0.060". Secondary cell over Harriet zone 0405 (Brush Creek 0.040"/0.024" 1-hr) and ungauged Davis Creek (0506: 0.046"). All values well below the ~0.3–0.4" empirical detection threshold established earlier in the study.
None of the seven gauges showed a meaningful rise. All are in continued recession, with all seven advancing study-period lows again today: - Boxley: new low 2.03 ft (-0.08 ft / 24 hr). - Pruitt: new low 3.63 ft / 90.3 cfs intraday; ended 3.66 ft / 96.2 cfs. First crossing into Too Low band (< 100 cfs threshold) — touched 90.3 cfs at 22:45; oscillating between high-Too-Low and low-Floatable through evening. - St. Joe: new low 3.74 ft / 272 cfs intraday; ended 3.75 ft / 279 cfs. -42 cfs / 24 hr from D75 end. - Harriet: new low 3.91 ft / 352 cfs intraday; ended 3.92 ft / 358 cfs. -73 cfs / 24 hr from D75 end. - Richland: new low 1.41 ft (-0.12 ft / 24 hr). - Bear Creek: new low 2.12 ft / 15.0 cfs (-0.03 ft / 24 hr). - Ponca: This needs special treatment — see anomaly section below.
None. Today's QPE was sub-threshold everywhere (max zone-avg 0.065" Ponca; max single-HUC12 0.072" Beech Creek). No detectable gauge rise attributable to today's rain. This is consistent with the established detection threshold (~0.3" zone-avg appears to be the floor for measurable mainstem response). Today reinforces this non-detection pattern.
N/A — no propagating signal today.
N/A — Event 10 fully resolved D74. Today is Day 4-5 post-final-peak recession.
Recession rate update — Comparing today's rates to the cross-event recession model: - St. Joe: 321 → 279 cfs over 22.75 hr ≈ -1.85 cfs/hr (Day 4 post-Event-10 final). Continues the geometric decay: Event 10 sequence at this gauge is now Day 1 (~?), Day 2 (-3.1 cfs/hr from D75), Day 3 (-1.85 cfs/hr today). Decay ratio ~0.60, consistent with the 0.63–0.73 range observed Event 9. - Harriet: 414 → 358 cfs over 23.75 hr ≈ -2.36 cfs/hr (Day 2-3 post-Event-10 final). Event 10 sequence: Day 1 (-4.3 cfs/hr D75), Day 2 (-2.36 cfs/hr today). Decay ratio ~0.55. Slightly faster decay than Event 9 (which was 6.3 → 4.4), possibly reflecting drier soils dampening sustained interflow contribution.
MAJOR — Ponca rating-curve discontinuity at D76 11:00. Ponca discharge dropped from 154 cfs @ 10:45 to 91 cfs @ 11:00 — a 63-cfs / 41% step-decrease in 15 minutes, with no rainfall, no upstream signal, and inconsistent with all other gauges. From 11:00 onward Ponca shows a new baseline of ~87-91 cfs, while Boxley (no rating curve, just height) shows smooth continued recession. This is almost certainly a USGS field calibration / rating-curve update rather than a hydrologic event. Local knowledge explicitly warns: "CFS readings can lag reality significantly... February 18th, the Ponca gauge CFS was off by approximately 50% during an initial river rise. The rating curve was not updated by USGS field teams for 48 hours." The 41% downward correction today is consistent with that warning — Ponca had likely been over-reporting at low flow.
Implications: - All Ponca cfs values reported across the study prior to D76 11:00 should be flagged as potentially over-reported by ~30-40% at low flows. This explains some prior puzzles, e.g., Ponca's seemingly-elevated low-flow values relative to Pruitt despite karst losses noted in local knowledge. - Cross-gauge comparisons involving Ponca cfs (Boxley→Ponca transfer ratios in cfs, Ponca→Pruitt loss estimates) carry larger uncertainty than previously assumed. Future Event-X transfer ratios at Ponca should use post-D76 11:00 rating curve as the baseline; prior values are not directly comparable. - Ponca is now firmly in Too Low (87.7 cfs vs 150-cfs threshold) under the new rating — a significant recreational state change driven by gauge calibration, not by flow change.
Pruitt enters Too Low band. Pruitt 90.3 cfs at 22:45 / 96.2 cfs at end of day. The Pruitt threshold is <100 cfs Too Low. Forecast from D75 (24-48 hr to threshold) was accurate. This is the first mainstem gauge to cross into Too Low in the study.
Confidence: high on both findings.