Changes flow regime downstream (decreases variability):

• prevents flooding.
• active flood plain and or channel system can narrow (with associated loss of riverine habitat).
• floods no longer act as recharge events for the shallow alluvial aquifer.
• flood silts are no longer deposited on flood plain.

Changes water chemistry and temperature downstream:

• exit water typically colder and lower or depleted in oxygen, with a higher organic content, and possibly hydrogen sulfide gas.
• can cause change in biota.
• can decrease the natural biodegradation capability of the river.
• somewhat increased salinity of waters due to the evaporation.

Loss of water from reservoir basin due to ground seepage:

• examples of where the dam basin so porous (due to cave systems and karst terrane) that couldn't fill the reservoir up. Cold Brook Dam, built in 1960 just outside of Hot Springs, South Dakota is one. There are three companion dams in the Black Hills built on the same unit which are unfilled. Source Rahn, 1986, Engineering Geology, Elsevier Press.
• Lake Powell formed by the Glen Canyon dam may lose up to 33% of its storage capacity per year to seepage into the underlying Navajo sandstone.
• recharges local aquifer.
• can lead to dam instability if dam is not built properly.
• with time muds can seal off dam basin and seepage decreases.

Loss of water due to evaporation:

• Lake Powell is estimated to lose 10% of its capacity per year to evaporation.
• function of local climate and surface area.

Changes sediment load downstream (decreases it), serving as a sediment trap.

• can cause widening of river channel for several miles downstream.
• can cause river downcutting.
• may decrease shoreline sand budget.
• eventual the dam can silt up. Typically a delta builds at the upper end of the reservoir.
• What determines the life span of a dam (before it silts up)?
  • incoming sediment load of river.
  • shoreline erosion.
  • reservoir volume.
  • measured annual loss of storage capacities due to sedimentation varies from .07 up to 1.2% for a series of dams in the U.S., with an average of .40 for 12 studied dams. How long before they fill up (given a simple minded linear extrapolation)?
  • West Pakistan Mangla Reservoir cost $600 million and filled up in 50 years before it was paid off.

Produces a shore line environment:

• this can include increased marginal vegetation that can increase loss through evapotranspiration.

Produces a lake environment, potentially increasing the biodiversity.

Changes load on the crust, potentially producing reservoir-induced seismicity.

• Oroville Dam, California -> 5.7 EQ in 1975.
• Koyna Dam near Bombay, India; reservoir infilling 1962, 103 m high, 2.8 billion cubic meters capacity. Built on Deccan basalt traps. 1967 6.4 RM EQ kills > 200, injures 2,300. EQs form N-S zone. additional dam built down stream - Warna dam, 1.3 billion cubic meters capacity. > 150 EQs > or = to 4.0 R. M. between 1962 and 1998.
  • Gupta, H. K., Radhakrishna, I. , Chadha, R. K., Kumpel, H. J. & Greksch, G., 2000, Pore Pressure Studies Initiated in Area of Reservoir-induced Earthquakes in India; EOS, 81, p. 145 & 151.
• Lake Mead has also produced earthquakes, as have reservoirs in South Carolina.

Dam safety issues:

• case history of Vaoint, Italy.
• Teton dam in Idaho, built in 1976 by U.S. Bureau of Reclamation, earth fill dam 94m high, failed upon filling. Killed 14 people and caused $1 billion worth of damage. Peak discharge estimated at 57,000 cubic meters per second. Failed because of abundant fractures in the foundation. The problem was recognized and attempts to fix it (removal and grouting) made, but they were inadequate. Geologists had warned that the site was not favorable. Link to collection of photos of results.
• Johnstone Flood May 31, 1889, south fork of the Conemaugh River in PA.
• 1981 inspection of 8639 high-risk (high cost if fails) dams by Army Corp of Engineers 2884 were found unsafe, and inadequate spillways accounted for 2351 of that number. Source Rahn, 1986, Engineering Geology, Elsevier Press.

• USGS report on Dams and Reservoirs on the Upper Missouri River.
• Geomorphic Effects of Overflow Dams on the Lower Neosho River, Kansas By Kyle E. Juracek (Kyle is a UNO graduate).
• Pros and cons of dam project from China Online.
• PBS report on the Three Gorges Dam Project.
• People's Daily on the Three Gorges Dam Project including recent updates.

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