Sediment Deficit and the Gorge Dam
The Upper Kelso catchment (840 km², mean annual precipitation 1,150 mm) feeds a regulated river. Between 1990 and 2010, upstream reforestation and 37 check dams were installed. At the downstream gauging station, annual peak discharge rose 18%, time-to-peak fell from 14 to 9 hours, and suspended sediment concentration fell 35%. Since 2005 the channel bed at the highway bridge has incised 1.2 m; gravel bars have disappeared and two banks now fail each year. A 60 m hydropower dam is proposed at the gorge: 45 MW, sediment trap efficiency 85%, designed reservoir life 35 years, minimum environmental flow 3 m³/s. Two villages (population 1,400) would lose riverside gardens. The regulator asks whether the dam will reduce or accelerate downstream incision over 30 years and what evidence should be required before approval.
1. Explain why suspended sediment concentration has fallen while downstream incision has accelerated. Identify two measurements that would distinguish between the competing explanations.
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Suspended sediment and bedload are not the same signal. Check dams and reforestation can reduce fine sediment from hillslopes, lowering suspended concentration, while check dams also trap coarse bedload. Downstream, the channel receives clearer water with a sediment deficit; if transport capacity is maintained, the flow entrains bed and bank material, producing incision.
The 18% rise in peak discharge and the shorter lag are the anomaly. Reforestation normally lowers peaks, so I would not accept the simple story. Possible explanations are increased rainfall intensity, compacted access tracks, road ditches, or a changed rating curve after incision.
Two decisive measurements:
- Bedload flux and grain size at the downstream station, paired with repeat channel cross-sections, to show whether coarse supply has fallen and whether the bed is lowering.
- Sub-hourly rainfall and runoff for at least 10 storms, to test whether the peak increase is climatic or land-management driven.
I would also check the gauging station rating curve, because a shifted stage-discharge relation can create an apparent peak trend after bed incision.
2. Assess whether the dam will alleviate or worsen downstream incision over 30 years. Use sediment-budget reasoning and state the conditions under which your judgement would change.
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The dam is likely to worsen incision for most of its 35-year life unless sediment is bypassed. With 85% trap efficiency, only 15% of the sediment load continues downstream; the clear release will have excess transport capacity, so the reach below the gorge will erode bed and banks.
Reduced peak flows may partly offset this by lowering transport capacity, but a minimum flow of 3 m³/s is a low-flow rule, not a sediment-management rule. The reservoir life of 35 years also implies a sediment store that will eventually need flushing or decommissioning; a poorly managed flush can send a damaging pulse downstream.
My judgement would change if the design includes year-round sediment bypass, gravel augmentation sized to the pre-dam bedload, or a flow regime that mimics pre-dam channel-forming discharges. Approval should be conditional on a sediment budget that closes: measure bedload input, trap efficiency by grain size, and downstream export. The two villages' gardens are a separate social impact, not evidence about incision.
3. Recommend a monitoring and adaptive-management plan with three indicators and explicit decision thresholds. Explain why each threshold is defensible.
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Indicator 1: bed elevation at five fixed cross-sections, surveyed twice yearly. Threshold: incision greater than 0.20 m at any section over any 24-month period, or 0.35 m at the bridge. This is defensible because it is about one-fifth of the 1.2 m observed since 2005 and precedes bridge scour risk.
Indicator 2: bedload and suspended-sediment flux at the downstream station, sampled monthly and during floods. Threshold: bedload flux below 60% of the pre-dam baseline for two consecutive years. This links directly to the sediment-deficit mechanism and gives time to act before irreversible incision.
Indicator 3: bank-failure inventory along 10 km, using fixed photo points. Threshold: more than two new failures per year or loss of more than 500 m² of bank. This protects roads, gardens and riparian habitat.
Adaptive actions: if threshold 1 is crossed, add gravel below the dam; if threshold 2, test sediment bypass or flushing; if threshold 3, reduce peak ramping and restore riparian vegetation. The plan needs a pre-dam baseline and an annual public report, otherwise thresholds become arbitrary.