Ground-Water Hydrology of the Upper Klamath Basin, Oregon and ...

Ground-Water Hydrology of the Upper Klamath Basin, Oregon and California
Purpose and Scope
This report summarizes the present understanding of
regional ground-water flow in the upper Klamath Basin
resulting from the USGS–OWRD cooperative study. The
report provides a description of the ground-water hydrology
for resource managers, water users, and those with a general
interest. It is also intended to provide sufficient quantitative
information useful to hydrologists working in the basin, and to
provide background for subsequent reports and investigations.
The report is regional in scope. Thus, data and analysis
presented herein are intended to provide an understanding of
the ground-water hydrology at a regional scale. Although the
information and interpretations are useful for understanding
the hydrologic setting of smaller areas, the ground-water
hydrology of local areas is generally not discussed in detail.
The report covers many key aspects of the regional
ground-water system. A summary of the geologic framework
of the regional ground-water system is presented that is
based on existing geologic mapping, field reconnaissance,
radiometric age dates, and interpretation of lithologic data
from more than 1,000 wells. A regional ground-water
budget (including recharge and discharge), calculated
from precipitation data, streamflow data, measured and
estimated ground-water discharge to streams, and estimated
evapotranspiration, also is provided. The distribution of
hydraulic head, which controls ground-water flow directions,
was estimated from water-level measurements in more than
1,000 wells, altitudes of gaining stream reaches, and altitudes
of hundreds of springs mapped on USGS quadrangle maps.
The response of the flow system to climate variations and
pumping stresses was determined by using time-series waterlevel
measurements from wells throughout the basin, with
frequencies ranging from hours to months and periods ranging
from years to decades, and from streamflow data.
Geologic Framework
Geologic Controls on Ground-Water Flow
The principal geologic factors that influence ground
water are the porosity and permeability of the rock or
sediment through which it flows. Porosity, in general terms,
is the proportion of a rock or deposit that consists of open
space. In a gravel deposit, this would be the proportion of the
volume of the deposit represented by the space between the
individual pebbles and cobbles. Permeability is a term used
to describe the ease with which a fluid can move through
a material such as rock or sedimentary deposits. Deposits
with large interconnected open spaces, such as gravel, offer
little resistance to ground-water flow and are, therefore,
usually highly permeable. Rocks with few, very small, or
poorly connected open spaces offer considerable resistance to
ground-water flow and, therefore, have low permeability. The
hydraulic characteristics of geologic materials vary between
and within rock types. For example, in sedimentary deposits
the permeability is a function of grain size and the range of
grain sizes (the degree of sorting). Coarse, well-sorted gravel
has much higher permeability than well-sorted sand. A wellsorted
sand or gravel has a higher permeability than a deposit
that is poorly sorted and has the open spaces between pebbles
or sand grains filled with silt or clay. Clay-rich deposits
generally have very low permeability. The permeability of
lava flows also can vary markedly depending on the degree of
fracturing. The highly fractured, rubbly zones at the tops and
bottoms of lava flows and in interflow zones are often highly
permeable, whereas the dense interior parts of lava flows
can have very low permeability. Weathering and secondary
mineralization, which often are a function of the age of
the rock, can strongly influence permeability. Sedimentary
deposits or lava flows in which the original open spaces
have been filled with secondary minerals can have very low
permeability.
Geologic properties that influence the movement of
ground water within a flow system also can define the
boundaries of the system. Geologic terranes consisting of
predominantly low-permeability materials can form the
boundaries of a regional flow system.
General Geology
The upper Klamath Basin has been a region of volcanic
activity for at least 35 million yr (years) (Sherrod and Smith,
2000), resulting in complex assemblages of volcanic vents
and lava flows, pyroclastic deposits, and volcanically derived
sedimentary deposits. Volcanic and tectonic processes have
created many of the present-day landforms in the basin.
Glaciation and stream processes have subsequently modified
the landscape in many places.
The upper Klamath Basin lies within two major geologic
provinces, the Cascade Range and the Basin and Range
Province (Orr and others, 1992). The processes that have
operated in these provinces have overlapped and interacted
in much of the upper Klamath Basin. The Cascade Range
is a north-south trending zone of compositionally diverse
volcanic eruptive centers and their deposits extending from
northern California to southern British Columbia. The
Cascade Range is subdivided between an older, highly eroded
Western Cascades, and a younger, mostly constructional
High Cascades. Prominent among the eruptive centers in the
High Cascades of the Klamath Basin are large composite
and shield volcanoes such as Mount Mazama (Crater Lake),
Mount McLoughlin, and Medicine Lake Volcano. The
Cascade Range has been impinged on its eastern side by the
adjacent structurally dominated Basin and Range Province.
The Basin and Range Province is a region of crustal extension
characterized by subparallel, fault-bounded, down-dropped