Run-of-River Hydropower

Summary

Nepal’s power system is dominated by run-of-river (RoR) hydropower: plants that divert part of a river through headworks and a tunnel, then return it downstream without holding a seasonal inventory of water. RoR is not “small hydro” and not automatically low-impact—it is an architecture defined by what you do not store.

Simple Explanation

Run-of-river hydropower generates electricity from the river water flowing past it right now, without building a big reservoir. Think of it as a water wheel on a large scale: when the river is full, it produces a lot; when the river is low, it produces little. In Nepal, this means most hydropower generates huge amounts in the monsoon and much less in winter.

RoR vs PRoR vs storage

Architecture	Water use	Seasonal carryover	Typical role
RoR	Uses contemporaneous river flow	None	Energy follows the hydrograph
Peaking RoR (PRoR)	Short pondage (hours) for shifting output within a day	Not seasonal storage; usually 2–6 hours of useful peaking	Better intraday shape, still hostage to the monsoon
Reservoir / storage	Large impoundment	Yes—wet-season inflow can be banked for dry-season release	Seasonal firming and strategic reserves

Fleet composition matters analytically: Nepal’s installed hydro is roughly ~85.7% RoR, ~9.9% PRoR, and ~3.7% storage (see Hydropower Fleet Composition). That single sentence is already a forecast: most megawatts are structurally aligned with wet-season inflow, not with steady demand.

Why Nepal favored RoR

The political economy pushed toward RoR for coherent reasons: lower unit cost than large storage, faster financial close for private IPPs, less resettlement than big reservoirs, and a regulatory/licensing path that matched export-oriented and domestic “capacity addition” narratives. RoR also fragments risk across many sponsors instead of concentrating it in one megaproject state balance sheet.

The trade-off is strategic: RoR solved how to add MW quickly without solving how to add seasonal value. More RoR in a monsoon climate does not average out into “more reliable power”—it tends to deepen the same dual outcome: surplus when the river is high and deficit when it is low (see Seasonal Mismatch).

Fleet type as destiny

In the medium term, fleet type is destiny: you cannot wish PRoR pondage into seasonal storage, and you cannot dispatch what physics never impounded. The rare counterexamples—Upper Tamakoshi as a comparatively “wet-season heavy” large RoR story, and Kulekhani Cascade as almost the entire operational storage base—illustrate how narrow the exceptions are.

For planning, the operative question is not headline megawatts but what kind of electricity those megawatts become across months: that is the bridge from RoR economics to Firm Power and the physical gap summarized as Storage Deficit.

Common Misunderstandings

"RoR = small hydro" — RoR is an architecture, not a size category. Large plants like Upper Tamakoshi (456 MW) are RoR.
"RoR provides continuous power" — output follows the hydrograph and collapses in dry season. There is nothing continuous about January output.
"Peaking RoR (PRoR) is seasonal storage" — PRoR pondage typically stores 2–6 hours of water, not weeks or months. See PRoR Is Not Seasonal Storage.

Seasonal Mismatch — the structural problem RoR amplifies
Firm Power — what RoR cannot deliver in dry months
Storage Deficit — the gap that explains why RoR dominance is a problem
Hydropower Fleet Composition — the numbers behind the architecture shares
Operating Fleet Is Overwhelmingly RoR/PRoR, Not True Storage — the canonical claim (>90% RoR/PRoR)
PRoR Is Not Seasonal Storage — why PRoR is often mistaken for storage