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If air temperatures are warmer than water temperatures, heat energy is transferred from air to water and
<br />the water warms. Since 1994, water temperatures at Diamond Creek (river mile' (RM) 226, or 386 km
<br />below the dam) averaged 7.5 °C warmer relative to releases in July but only 4 °C warmer in October
<br />(Voichick and Wright, 2007; Wright and others, 2008). If air temperatures are cooler than water
<br />temperatures, then heat energy is transferred from water to air and the water cools; in December, water
<br />temperatures at Diamond Creek are on average slightly cooler ( <PQ than dam release temperatures.
<br />The amount of mainstem warming or cooling that occurs is positively related to water residence time,
<br />and the primary determinant of residence time is discharge volume (residence time decreases as
<br />discharge volume and average velocity increase). For example, water temperatures at Diamond Creek
<br />were 10 °C warmer relative to releases in June 2000, when discharge was a constant 8,000 cubic feet per
<br />second (cfs), but only 5 °C warmer during June of 1997, when discharge was a relatively constant
<br />25,000 cfs (Vernieu and others, 2005).
<br />If the residence time of water is sufficiently long, water temperatures of nearshore habitats can be
<br />different from the mainstem. For example, Korman and others (2006) found that water temperatures
<br />along high -angle shorelines —where water velocity is fast and residence time is short—were no different
<br />than the mainstem river across all months studied (mid- August through mid - November of 2004; three
<br />reaches were sampled: RM -3.5, RM 44.6, and RM 64.5). In contrast, daytime water temperatures in
<br />backwaters (longest water residence time) and low -angle shorelines (intermediate residence time) were
<br />about 2 and PC warmer, respectively, relative to mainstem temperatures in August. In September,
<br />when air temperatures were cooler but water residence time presumably was longer because of lower
<br />discharge volume and fluctuation range (10,000 to 18,000 cfs in August versus 5,000 to 10,000 cfs in
<br />September and October), water temperatures in backwaters and low -angle shorelines were about 3 and
<br />2 °C warmer than mainstem temperatures, respectively. However, in October, average daily air
<br />temperatures were cooler than water temperatures, and, as a result, water temperatures in these habitats
<br />were the same or slightly cooler than the mainstem. Thus, September and October represent a period of
<br />transition during which nearshore warming declines and eventually ceases, at least when discharge is
<br />fluctuating.
<br />Mainstem and nearshore water temperatures for 42 miles of the Colorado River (RM 30 to 72) were
<br />measured using an aerially deployed thermal infrared sensor on July 21, 2000, around 1 p.m. local time
<br />as part of the low summer steady flow (LSSF) experiment (B. Ralston, U.S. Geological Survey, written
<br />commun., 2009). Resolution of these data were 1 m2 and the total area of habitat within 2 and 4 in of
<br />shore were 446 ha and 804 ha, respectively. Water temperatures were warmer closer to the shore and
<br />ranged from 9 to 30 °C (mainstem temperatures in this reach were about I YC at the time), indicating
<br />significant nearshore warming can occur during the summer months when discharge is constant.
<br />However, 67 percent and 75 percent of the nearshore habitat within 2 and 4 meters of the shoreline,
<br />respectively, had water temperatures that were actually the same or slightly cooler than the mainstem.
<br />Unfortunately, thermal infrared images providing a comprehensive picture of nearshore water
<br />temperatures have not been collected during fluctuating operations at the same time of year as the low
<br />summer steady flow experiment, so it is difficult to put these data into context. That is, it is unclear
<br />whether nearshore habitats will gain increased water temperatures during steady flows relative to
<br />fluctuating flows.
<br />' By convention, river mile (RM) is used to describe distance along the Colorado River in Grand
<br />Canyon: Lees Ferry (located 15.7 miles downstream of Glen Canyon Dam) is the starting point, as RM
<br />0, with mileage measured for both upstream ( -) and downstream directions.
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