Sources and Characterization of Particles Affecting Transparency
in Grand Lake and Shadow Mountain Reservoir
Prepared by: James H. McCutchan, Jr. June 10, 2015
Rpt 339 Table of Contents Introduction 1 Background Information on Grand Lake and Its Water Sources 3 Physical and Chemical Characteristics of the Lakes in 2012 5 Hydrology 5 General Characteristics of Grand Lake and Shadow Mountain Reservoir: 2012 13 Background Information on Transparency in Lakes 30 Measurement of Transparency and Other Optical Properties of Water 30 Factors Affecting Transparency 33 The Transparency Record 51 Secchi Transparency 52 Attenuation Coefficients 56 Turbidity 59 Separation of Transparency Components 63 Dissolved Organic Matter 63 Phytoplankton 63 Non-Algal Particles 64 Spectral Interactions 64 Regression Analysis of Variation in Transparency 71 Sources and Sinks of Particles 79 Import of Particles by Tributaries 79 In Situ Production of Particles 85 Sinks for Particles 90 Mass Balance of Particles 96 Stable Isotope Ratios 102 Conclusions about Transparency 108 Primary Controlling Factors 109 Implications for Management Alternatives 116 References 119 Appendix I. 121
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Summary
Northern Water, Grand County, and the U.S. Bureau of Reclamation sponsored a multi- year study of particles affecting transparency in Grand Lake and Shadow Mountain Reservoir.
This report summarizes and interprets information that was collected as part of the study. Results of this study will be used to support the development of appropriate management and control measures to improve transparency in Grand Lake and Shadow Mountain Reservoir.
This study included three main components: routine monitoring, mapping studies, and watershed studies. Routine monitoring included field measurements, sampling, and analyses for locations on Shadow Mountain Reservoir, Grand lake, and their tributaries at points near the lakes. Mapping studies (2012 and 2014) included field measurements across many locations on the lakes. Watershed studies (2013 – 2014) included field measurements, sampling, and analyses for multiple locations within the Colorado River watershed, as necessary to identify locations of particle sources. Variation in hydrologic conditions over the study period has provided an opportunity to study controls on transparency over a wide range of conditions.
Grand Lake is strongly affected by its native water sources during snowmelt runoff, but its characteristics typically change after peak runoff due to the introduction of water from
Shadow Mountain Reservoir. The changes include higher specific conductance, higher nutrient concentrations, and higher particle concentrations. The effect of pumped water on Grand Lake was less in 2013 and 2014 than in 2012, but Grand Lake still was strongly affected in 2013 and
2014 because the effects of pumping persist even after pumping has stopped.
The optical properties of natural waters are determined by the combined optical properties of water molecules, dissolved substances, and suspended particles. As light passes
ii through water, photons are lost through absorption, and scattering increases the probability of absorption. Transparency is the property of allowing transmission of light without absorption or scattering.
Transparency in Grand Lake and Shadow Mountain Reservoir was measured as Secchi transparency, which is the depth within the water column to which a reflective disk remains visible to an observer at the surface. Attenuation coefficients were determined from profiles of irradiance (photon flux) over a range of wavelengths with an underwater quantum sensor. With these measurements of transparency and measurements of dissolved and suspended substances in individual water samples, different approaches were used to determine the effects of individual components affecting transparency. Results show that transparency in the lakes is determined largely by concentrations of particles in surface water. In general, non-algal organic particles had the greatest effect on transparency, but inorganic particles and algal particles also were important.
Chromatic dissolved organic matter affects transparency but accounts for only a small portion of variance. For a given mass of particles, organic particles (algal particles, non-algal organic particles) had a greater effect on transparency than inorganic particles, but the overall effect of inorganic particles on transparency was only slightly lower than the overall effect of non-algal organic particles or algal particles. The dominant sources of non-algal organic particles, as determined by analysis of stable isotope ratios, were the Granby Pump Canal and the native tributaries. Macrophytes, although abundant in Shadow Mountain Reservoir, were not an important source of suspended particles, and the abundance of macrophytes may increase, rather than decrease, transparency in the lakes.
Particles transported by the native tributaries and the Granby Pump Canal often dominate the suspended particles in Grand Lake and Shadow Mountain Reservoir. For the native
iii tributaries, particle concentrations tend to increase with discharge, especially for the North Fork of the Colorado River. When flows in the native tributaries are low, the Granby Pump Canal can be the major source of particles reaching the lakes, but concentrations of particles in the Pump
Canal are much lower than the highest concentrations carried by the North Fork of the Colorado
River. Thus, water carried by the Granby Pump Canal sometimes is a source of dilution for more concentrated sources of particles (e.g., the North Fork of the Colorado River at times of high discharge).
Four primary factors ultimately control most of the variation in particle concentrations in
Grand Lake and Shadow Mountain Reservoir (Table A). For a given set of conditions, it is possible to predict particle concentrations and transparency in the lakes (Table B). For any set of conditions that produces concentrations of suspended particles greater than ~2.5 mg/L, Secchi transparency is not likely to exceed 4 m. However, because of the stochastic nature of the primary factors affecting particle concentrations, there is considerable variation in transparency for any given time of year, and precise predictions of transparency are possible only over short periods of time.
Identification of the primary factors affecting particle concentrations and transparency in
Grand Lake and Shadow Mountain Reservoir provides information on possible management alternatives that could be used to improve Secchi transparency in the lakes (Table C). Some of the management alternatives listed in Table C may not be feasible, and some would have unwanted consequences on water quality in Shadow Mountain Reservoir or elsewhere in the C-
BT system.
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Factor Mechanism Native flows Particle conc. for tributaries increase with discharge; WRT decreases Season (time of year) Temp. affects algal growth, mixed depth; snow cover affects particle yield Lake depth Resuspension of particles; internal loading of nutrients Farr pumping Pumping changes water source for SMR, GL; affects WRT, mixed depth
Table A. Primary factors affecting particle concentrations and transparency in Shadow Mountain Reservoir and Grand Lake. WRT = water residence time; SMR = Shadow Mountain Reservoir; GL = Grand Lake.
Factors Characteristic particle Characteristic Secchi Native Farr concentration, mg/L transparency, m flows Season pumping SMR GL SMR GL High Warm On 5 3 1.5 2 Low Warm On 2.5 2.5 2.5 2.5 High Cool On 3 2 2 3 Low Cool On 2.5 2.5 2.5 2.5 High Warm Off 5 2 2 3 Low Warm Off 8 1 1 4 High Cool Off 3 2 2 3 Low Cool Off 3 <1 2 5 - 8
Table B. Characteristic particle concentrations and Secchi transparency in Shadow Mountain Reservoir and Grand Lake for different combinations of factors under current operations of the C-BT system. Secchi transparency is estimated here from TSS, according to the relationship given in Figure 56. Shading indicates sets of conditions with Secchi transparency outside the range of ~2 – 4 m that is typical under present operations of the C-BT system.
Factor Management alternatives Native flows Diversion of Colo. R. at high flow (e.g., via Redtop); sedimentation basins Lake depth Control sources of suspended particles in SMR; deepen Shadow Mtn. Res. Season (time of year) Alteration of GL mixed depth; nutrient control of phytoplankton growth Farr pumping Major alteration of C-BT; operational control; line Pump Canal
Table C. Some management alternatives corresponding to primary factors affecting transparency.
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Introduction
Suspended particles, including suspended algae, contribute substantially to light attenuation in Grand Lake and Shadow Mountain Reservoir. A study conducted in the summer of 2009 showed that the concentration of non-algal suspended particles was the most important factor affecting transparency in both lakes at that time (McCutchan 2010). However, the relative importance of algal versus non-algal particles varies seasonally and across years, and the sources and composition of non-algal suspended particles were not well resolved.
From the results of the 209 study, it was apparent that a clear understanding of the effects of algal and non-algal particles on transparency in Grand Lake and Shadow Mountain Reservoir under present and future operations would require the study of particles in both lakes and their water sources, including North Inlet, East Inlet, the North Fork of the Colorado River, and the
Granby Pump Canal. Storm runoff not associated with tributaries also may be important. In addition to external sources of particles, phytoplankton and aquatic macrophytes add particles to the water column, as does shoreline erosion. Also, particles that settle in shallow water (e.g., on deltas or in the north end of Shadow Mountain Reservoir) can be resuspended.
Northern Water, Grand County, and the U.S. Bureau of Reclamation sponsored a multi- year study of particles affecting transparency in Grand Lake and Shadow Mountain Reservoir.
Field studies were conducted from May 31, 2012 through September 8, 2014. The same methods used in the 2009 transparency study (McCutchan 2010) were used for this study, but some other methods were added to provide additional information on sources and characterization of particles. The 2012 – 2014 Particle Study included three main components, as follows: routine monitoring, mapping studies, and watershed/stormflow studies. Routine monitoring included field measurements, sampling, and analyses for locations on Shadow
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Mountain Reservoir, Grand Lake, and their tributaries at points near the lakes. Mapping studies
(2012 and 2014 only) included field measurements across many locations on the lakes.
Watershed studies (2013 – 2014) included field measurements with sampling and analyses for multiple locations on the North Fork of the Colorado River and its tributaries, as necessary to identify locations of particle sources within the Colorado River watershed. The first year of the study, 2012, was a dry year, with pumping from Granby Reservoir to Shadow Mountain
Reservoir over the entire summer. The early part of 2013 was dry, but late season snow resulted in near-average snowmelt runoff and pumping was stopped experimentally for six weeks during the summer in 2013. Pumping also was halted after a storm in September, 2013. Snowmelt runoff in 2014 was higher than average; there was very little pumping in 2014 between mid May and late July, and pumping was stopped for most of August. Thus, the three years of this study have provided an opportunity to investigate controls on transparency in the lakes over a wide range of hydrologic and operational conditions. Sampling stations and dates of sampling are listed in Appendix I.
The main purpose of this study was to identify and characterize sources of suspended particles in Shadow Mountain Reservoir and Grand Lake over the study period. Results of this study will be used to support the development of appropriate management and control measures to improve transparency in Grand Lake and Shadow Mountain Reservoir and to guide future research and monitoring.
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Background Information on Grand Lake and its Water Sources
Grand Lake (8367 feet amsl) is Colorado’s largest and deepest natural lake. Originally, it received water from North Inlet Creek and East Inlet Creek (Figure 1, Table 1), smaller streams, and groundwater seepage. The outflow was west, to the Colorado River. Shadow Mountain
Reservoir, which was completed in 1946 as part of the Colorado-Big Thompson (C-BT) Project, impounds the flow of the North Fork of the Colorado River, the outlet of Grand Lake, and
Columbine Creek adjacent to Grand Lake. Operation of Shadow Mountain Reservoir stabilizes the level of Grand Lake, which by law cannot be altered by more than one foot. The Granby
Pump Canal joins Shadow Mountain Reservoir at its southern end, allowing water to be pumped from Granby Reservoir (completed in 1951) to Shadow Mountain Reservoir. The short channel connecting Shadow Mountain Reservoir to Grand Lake allows flow from Shadow Mountain
Reservoir, much of which originates from the Granby Pump Canal, to displace water from
Shadow Mountain Reservoir into Grand Lake, or the reverse if water rises in Grand Lake. Water in Grand Lake can be released to the Adams Tunnel (Figure 1: north of EI-GLU), which serves water rights on the Eastern Slope of Colorado. Thus Grand Lake, which previously received no water from lower elevation, now receives water seasonally from Granby Reservoir, Shadow
Mountain Reservoir, and their tributaries (e.g., diversions from Willow Creek Reservoir and the
Windy Gap Project).
The three lakes that now contribute to flow at the Adams Tunnel differ considerably in their morphometric characteristics. Grand Lake is the smallest of the three by area, but has high relative depth. Granby Reservoir has the greatest surface area and the greatest volume of the three lakes. Shadow Mountain Reservoir is intermediate in size but shallow. Granby Reservoir and Grand Lake would be expected to develop a fully formed three layer stratification system
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during summer (epilimnion, metalimnion, hypolimnion), whereas Shadow Mountain Reservoir would be expected to show at most temporary stratification and lack a fully formed hypolimnion.
Figure 1. Shadow Mountain Reservoir, Grand Lake, their water sources, and sampling locations for the 2012-2014 particle study.
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Shadow Granby Reservoir Grand Lake* Mtn. Res. Volume, acre feet 540,000 17,400 68,600 Surface area, acres 7,260 1,330 507 Maximum depth, ft (m) 220 (67) 25 (7.6) 265 (81) *The assumed maximum epilimnion volume (0-4.6 m) of Grand Lake is 3,977 acre feet.
Table 1. Physical characteristics of Lake Granby, Shadow Mountain Reservoir, and Grand Lake.
Physical and Chemical Characteristics of the Lakes
The Particle Study has incorporated some collection of data on physical and chemical variables in Grand Lake and Shadow Mountain Reservoir that are not expected to have direct effects on transparency. This information provides insight into physical structure of the water column, nutrient concentrations reflecting potential to produce algal biomass, and other related matters that may affect the interpretation of factors influencing transparency. These general water quality conditions are summarized and interpreted here.
Hydrology
A complete review of the hydrology of the three-lake system is beyond the scope of this report, but an overview of the native flows and flow management is important to the interpretation of factors affecting the transparency of Shadow Mountain Reservoir and Grand
Lake. Native flows reaching Grand Lake (North Inlet, East Inlet) and Shadow Mountain
Reservoir (North Fork of the Colorado River) show the expected seasonal runoff cycle (Figure
2). Flow data are shown for the two gages on the North Fork of the Colorado River; differences in flow between the two locations reflect gains from tributaries below the USGS gage, near
Baker Gulch, and withdrawals via the Redtop Valley Ditch (up to ~75 cfs) and Lehman Ditch
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Figure 2. Native flows reaching Shadow Mountain Reservoir and Grand Lake.
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(up to ~5 cfs). Peak flows of the North Fork (to Shadow Mountain Reservoir) and the East and
North Inlets (to Grand Lake) were similar during 2012, rising to 150-200 cfs in May or early
June, followed by a rapid decline to base flows below 50 cfs for the North Fork of the Colorado
River and below 25 cfs for North and East Inlets. Columbine Creek carries smaller flows to
Shadow Mountain Reservoir. The native flows contribute significantly to withdrawals through the Adams Tunnel during the early part of the irrigation season. Peak flows in 2013 (400 – 600 cfs) were higher than in 2012. For East Inlet and North Inlet, peak flows in 2014 were comparable to peak flows in 2013; for the North Fork of the Colorado River, peak flows in 2013
(>1200 cfs) were much higher than in 2012 or 2013. Storms in late July, 2012 caused notable increases in flow in the native tributaries, and another storm in September, 2013 caused large spikes in discharge, especially on North Inlet and East Inlet.
In 2012, which was characterized by low spring runoff, pumping from Granby Reservoir was continuous from mid June through the summer and into the fall. The Adams Tunnel, with its intake near the surface (spanning 3-4 m depth), withdrew water at ~500 cfs, as a daily average, from May through October, except for a two-week period in early June when pumping was stopped for maintenance (Figure 3). After pumping resumed in mid June, the Granby Pump
Canal supplied most of the water diverted by the Adams Tunnel. Snowmelt runoff was higher than average in 2013, and the native flows provided all of the water for the Adams Tunnel from mid May through late June. Hydrologic conditions in 2013 made it possible to stop pumping for a six-week period from late July through early September, and pumping was stopped again on
September 12, 2013 due to flooding on the East Slope. Snowmelt runoff was much higher in
2014 than in 2012 or 2013, and there was virtually no pumping from Granby Reservoir from mid
May until mid July and also for periods of time in August and October. There was some
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Figure 3. Pump and tunnel operation. The record of flow measurements by acoustic Doppler velocimeter (ADV; USGS) for the connecting channel (SMR to Grand Lake) is incomplete and calculated flows are used instead of measured flows for some dates.
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pumping in late July and early August, and the pumps were operated for most of October.
Pumping operations in 2014 resulted in several changes in the director of flow through the channel connecting Shadow Mountain Reservoir and Grand Lake (Figure 3). As expected, the flows estimated for the connecting channel correlate well with the flow in the Pump Canal, even though the two are separated by the length of Shadow Mountain Reservoir. The management system is designed to allow pumped water to flow freely from the Pump Canal intake to Grand
Lake via the connecting channel that joins Shadow Mountain Reservoir to Grand Lake.
The influence of a water source on the water quality of a lake is related to the volume of the source over a given period of time, relative to the volume of the lake or the lake layer that it enters. Hydraulic residence time, which is the ratio of lake volume or layer volume to lake inflow (in this case expressed as days) is a convenient metric for the relative influence of a water source on a lake or a layer of a lake over a specific period of time. Figure 4 shows hydraulic residence times for water from specific sources entering Shadow Mountain Reservoir and Grand
Lake. For Shadow Mountain Reservoir, the entire lake volume is used in the hydraulic residence time calculation. The lake is shallow and usually does not develop stable thermal layering during summer, except for brief periods of time. For Grand Lake, the hydraulic residence time refers in this case only to the upper mixed layer (epilimnion; approximately 0-4.6 m, as assumed in the Three Lakes Water Quality Model; AMEC 2008), because the deeper layers are not mixed with the incoming flows during the warm season. The mixed layer thickens substantially in late
September and October as Grand Lake cools, and thickness of the mixed layer can change dynamically over a 24-hour period, but 4.6 m is a reasonable approximation for the thickness of the mixed layer during summer.
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Figure 4. Hydraulic residence time for components of flow to Grand Lake and Shadow Mountain Reservoir. Shading indicates periods with no pumping from Granby Reservoir to Shadow Mountain Reservoir.
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For Shadow Mountain Reservoir, hydraulic residence time was moderately low during
May, 2012 because of runoff and pumped flow. During the period when pumping was stopped for maintenance (June 4 – 14, 2012), hydraulic residence time was long and inflow was dominated by native flow. When the pumps were turned on in mid June, residence time declined to about 15 days and remained at that level until the end of the study interval in late October. In
2013, hydraulic residence time was low or moderately low from May through late July, when pumping was stopped, due to runoff and pumped flow. Residence time was high during the six- week period with no pumping; residence time dropped briefly when pumping resumed in
September, 2013, but rose again after pumping was stopped later in the month.
In 2012, hydraulic residence time in Grand Lake was below 10 days for the epilimnion, except when pumping was stopped in June. Both pumped flow and native flow made contributions in May, but pumped flow was strongly dominant from mid June through October.
In 2013, hydraulic residence time for the epilimnion of Grand Lake was low from May through late July, when pumping was stopped. Residence time increased from about 20 days to more than 60 days over the six-week period with no pumping. Residence time dropped again when pumping resumed in early September and rose gradually after the September storm.
Table 2 summarizes the dominant water sources and changes in hydraulic residence time for the time intervals relevant to this study. Granby Reservoir sometimes is listed as a source for
Grand Lake, even though Grand Lake received its water from Shadow Mountain Reservoir through the connecting channel, as hypolimnetic water from Granby Reservoir was entering
Shadow Mountain Reservoir (the Granby intake is 26 m above bottom, 30 m below the surface when Granby is full) during periods when the pumps were operating.
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Shadow Mountain Reservoir Grand Lake* Residence Important Water Sources Residence Time Time Interval Time Water Sources 2012 Variable Pumping Runoff, SMR/Granby**, Moderate Very short 1 May – 3 Jun Granby** Runoff No Pumping Runoff Long Runoff Short 4 Jun – 14 Jun Consistent Pumping Granby** Short SMR/Granby** Very short 15 Jun – 31 Oct
2013 Variable Pumping Granby**, Moderate SMR/Granby**, Very short 1 May – 12 May Runoff Runoff No Pumping Runoff Moderate Runoff, Very short 13 May – 24 Jun SMR/Granby** Variable Pumping Granby**, Moderate SMR/Granby**, Very short 25 Jun – 23 Jul Runoff to short Runoff No Pumping Runoff Long Runoff*** Moderate 24 Jul – 2 Sep to long Variable Pumping Granby** Variable SMR/Granby** Very short 3 Sep – 12 Sep No Pumping Runoff Long Runoff Moderate 13 Sep – 1 Nov (storm) (storm)
2014 Variable Pumping Granby** Short SMR/Granby** Short 1 May – 18 May Runoff Runoff
No Pumping Runoff Very short Runoff Very short 18 May – 20 Jul to moderate
Variable Pumping Granby** Short to SMR/Granby** Very short 21 Jul – 2 Sep Runoff moderate Runoff to moderate
Variable Pumping Granby** Very short SMR/Granby** Very short 3 Sep – 27 Sep Runoff to short to short
No Pumping Runoff Moderate Runoff Very short 28 Sep – 23 Oct to moderate
Variable Pumping Granby** Very short SMR/Granby** Short to 23 Oct – 1 Nov Runoff to moderate Runoff moderate *Epilimnion **Hypolimnion ***Some flow from SMR prior to 8/9/2013
Table 2. Dominant water sources and hydraulic residence times for selected time intervals.
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General Characteristics of Shadow Mountain Reservoir and Grand Lake
Temperature: Temperature profiles for the SM-DAM station in Shadow Mountain
Reservoir during 2012 show no evidence of stable stratification over long periods of time (Figure
5). Even near the dam, where stratification is most likely, stable stratification over the entire summer was not expected because Shadow Mountain Reservoir is shallow and flow from the
Granby Pump Canal often reduces stability of the water column. Profiles for other stations on
Shadow Mountain Reservoir (not shown) were similar to the profiles for the upper water column at the dam station, but not identical. Deviations between the bottom and surface temperatures in late June and early July suggest the possibility of temporary stratification of Shadow Mountain
Reservoir near the dam, but the warmth of the water on the bottom of the reservoir shows that the bottom water was affected by mixing with overlying water. As expected, maximum temperatures for the epilimnion tends to occur in late July or August, although the warmest temperature at the surface in 2012 was in June. In 2013, when pumping was stopped, surface water in Shadow Mountain Reservoir was near 20 °C, and the temperature difference between surface water and bottom water suggested that the water column was stratified, at least temporarily. Deep-water temperatures well above 8 °C in both 2013 and 2014, as in 2012, indicate some exchange of water between the surface layer and the underlying water.
Temperature profiles show that Grand Lake has the characteristics of a dimictic temperate lake, as expected. The uppermost water column over the warm months, when most of the samples were taken, consisted of a relatively warm epilimnion (13 - 17 °C) and the lowermost part of the water column consisted of a hypolimnion with a temperature of ~4 °C, near the temperature of maximum density of water (Figure 5). The epilimnion (upper 2 - 7 m,
June - September) of Grand Lake was not as well defined as might have been expected. Also,
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Figure 5. Temperature profiles for Shadow Mountain Reservoir (SM-DAM) and Grand Lake (GL-MID).
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the metalimnion, or middle layer, was thicker than might be expected for a lake of this size and depth. High rates of water flow from points outside the watershed and withdrawal from 3-4 m via the Adams Tunnel probably reduced heat accumulation, thus reducing the stability of layering. The temperature profile for October 29, 2012 showed that the water column was mixed to a depth of more than 20 m. Stratification developed later in the year in 2013 and 2014 than in
2012. As a result of the six-week period with no pumping in 2013, surface temperatures during the warmest months were higher in 2013, and the depth of the mixed layer tended to be less than in 2012 or 2014.
Specific Conductance: Specific conductance is an indicator of dissolved solids content
(salinity, i.e., total ionic solids). The specific conductances of all C-BT waters in the study area were low, reflecting their origin from nearby high elevation sources dominated by snowmelt and without extensive soil contact or evaporation losses. Specific conductance is useful as an indicator water sources when conductances of sources differ.
The natural sources of water for Grand Lake had median specific conductances near 20
μS/cm, May through September. Specific conductance for the North Fork of the Colorado River was higher (70 – 120 µS/cm), as was conductance for water from Granby Reservoir (~60
µS/cm). The deepest water in Grand Lake also had moderately high conductance (~60 µS/cm) through the stratification season; this water was residual from the previous cool season and probably originated from Shadow Mountain Reservoir and Granby Reservoir, with some influence from the native tributary flows. In 2012, conductance was nearly constant with depth in Shadow Mountain Reservoir. In 2013, differences in conductance between surface and bottom water indicated stratification of the water column on some dates, including dates during the six-week period with no pumping (Figure 6). 2014 also showed variation in specific
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Figure 6. Profiles of specific conductance for Shadow Mountain Reservoir (SM-DAM) and Grand Lake (GL-MID).
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conductance with depth on some dates. In 2012, surface water in Grand Lake had low conductance in June but was replaced over the summer by water of higher conductance (Figure
6). Conductance of surface water also increased over the summer in 2013, but only slightly due to reduced pumping and higher native flows.
Dissolved Oxygen and pH: During the warm season, dissolved oxygen in dimictic lakes tends to be slightly above saturation in epilimnetic waters and somewhat depleted or completely depleted toward the bottom (hypolimnion). Except in September, slight supersaturation
(saturation is ~7 mg/L for Shadow Mountain Reservoir at 16 °C) occurred near the surface in
Shadow Mountain Reservoir in 2012, as expected with dominance of photosynthesis over respiration (Figure 7). Oxygen concentrations near the bottom were only slightly lower than near the surface. Pumping of hypolimnetic water from Granby Reservoir affects oxygen concentrations in Shadow Mountain Reservoir, and oxygen concentrations near the bottom were
~6 mg/L in September, 2012. When pumping was stopped for six weeks in 2013, dissolved oxygen concentrations near the surface were well above saturation and concentrations near the bottom fell to near zero by late August.
Profiles of dissolved oxygen in 2014 were similar to profiles in 2012, except that the dissolved oxygen concentration was <4 mg/L near the bottom at SM-DAM in August, as a result of pumping for Granby Reservoir and temporary stratification over the deepest part of the reservoir.
Oxygen concentrations near the surface of Grand Lake remained above saturation over the study period. Oxygen depletion occurred in the deeper water of Grand Lake because of respiration but depletion was mild due to the very large hypolimnetic volume and low productivity of Grand Lake.
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Figure 7. Dissolved oxygen profiles for Shadow Mountain Reservoir (SM-DAM) and Grand Lake (GL-MID).
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The waters of Grand Lake and the two nearby reservoirs are weakly buffered because the drainages provide only small amounts of bicarbonate. Under these circumstances, pH in equilibrium with the atmosphere will be near 7. Substantial positive deviations from 7 are typically caused by photosynthesis, and negative deviations are typically caused by respiration.
In 2012, Shadow Mountain Reservoir had high pH (> 8.5) in June, and pH was generally reflective of a predominance of photosynthesis (Figure 8). In 2013, pH at the surface was >9.0 during the six-week period with no pumping. The pH did not exceed 8.0 on the dates of sampling in 2014. As expected in a shallow reservoir, there was very little vertical differentiation in pH over the water column except when water residence time was high. Grand
Lake showed moderately high pH in the epilimnion from June to September, with little difference among the three years of the study. Maximum pH was near 8.5 in August, except in
2014 when maximum pH was slightly lower. The water quality standard for protection of aquatic life in the lakes is 9.0. Thus, the August pH values for Grand Lake in 2012 and 2013 can be considered high with reference to the standard, and some values for Shadow Mountain
Reservoir in 2013 exceeded the standard. These high pH values result from the influence of photosynthesis on waters that have very little natural buffering capacity. Waters below the epilimnion, which support little or no photosynthesis, were influenced by respiration, which caused the pH to drop slightly below 7. Flowing waters showed pH within the range of values for Grand Lake and Shadow Mountain Reservoir.
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Figure 8. Profiles of pH for Shadow Mountain Reservoir (SM-DAM) and Grand Lake (GL-MID).
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Nutrients: Nitrogen and phosphorus are the key nutrients limiting growth of algae in aquatic ecosystems. The growth of phytoplankton often is limited by one or the other of these two elements, especially toward the end of the growing season.
Phosphorus was analyzed as two fractions, total dissolved phosphorus (TDP) and particulate phosphorus (TPP). TDP includes dissolved organic phosphorus and soluble reactive phosphorus (SRP), which is primarily inorganic and is the form of phosphorus that is considered most available for uptake by algae. Because algae have external phosphatases that release phosphate from soluble organic compounds, TDP is considered to be available, but not quite so readily available as SRP. Particulate phosphorus consists of inorganic and organic components.
Typically in lakes, the organic component predominates for particulate phosphorus; in flowing waters, inorganic phosphorus associated with suspended mineral particles often predominates.
Summation of total dissolved phosphorus and particulate phosphorus gives total phosphorus.
As judged by total P, Shadow Mountain Reservoir and Grand Lake have high P concentrations with reference to background for montane waters (Figures 9 – 10, Table 3).
Montane lakes at high elevation in Colorado typically have phosphorus concentrations below 7
μg/L under natural conditions or with weak anthropogenic sources of phosphorus (e.g., Lewis et al. 1984). Shadow Mountain Reservoir shows phosphorus augmentation from the North Fork of the Colorado River and the Granby Reservoir hypolimnion (Granby Pump Canal). The North and East Inlets had P concentrations near background for post-runoff conditions but slightly higher during runoff. Total phosphorus concentrations in the epilimnion of Grand Lake often were similar to concentrations in Shadow Mountain Reservoir and were approximately 40-50% higher than native concentrations for Grand Lake (i.e., concentrations reflecting phosphorus carried by North Inlet and East Inlet).
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Figure 9. Concentrations of total dissolved phosphorus (TDP) in water sources and in surface water of Shadow Mountain Reservoir and Grand Lake.
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Figure 10. Concentrations of total particulate phosphorus (TPP) in water sources and in surface water of Shadow Mountain Reservoir and Grand Lake.
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Location Pump Colorado Shadow North East Grand Lake Canal* River Mtn Res Inlet Inlet 0-2 m 18-20 m 2012 Total dissolved P 8.1 5.7 4.9 3.6 5.3 4.5 3.8 Particulate P 3.2 2.9 5.7 1.4 1.7 4.0 2.1 Total P 11 8.6 11 5.0 7.0 8.5 5.9 Ammonium N 6.7 6.0 2.5 5.1 9.0 3.2 4.2 Nitrate N 24 12 <2 70 52 <2 47 Total dissolved N 178 98 131 157 190 151 159 2013 Total dissolved P 9.3 5.0 6.9 3.6 3.9 4.0 3.7 Particulate P 11.9 7.6 13.4 4.1 4.5 8.9 4.7 Total P 21.2 12.6 20.3 7.7 8.4 12.9 8.4 Ammonium N 7.5 6.0 16.1 6.0 6.3 7.6 9.8 Nitrate N 26 11 4.0 60 62 13 56 Total dissolved N 162 96 181 142 146 136 167 2014 Total dissolved P 8.1 5.2 5.1 3.7 3.7 3.5 3.9 Particulate P 7.9 6.0 9.3 3.5 2.7 6.0 3.4 Total P 16.0 11.1 14.4 6.4 7.2 9.5 7.3 Ammonium N 3.1 4.8 3.2 5.5 5.3 5.4 6.9 Nitrate N 41 16 7 53 43 14 62 Total dissolved N 179 128 158 161 147 143 206 * Canal not flowing on first sampling date in 2012; canal flowing on only three sampling dates in 2013 and two in 2014.
Table 3. Mean nutrient concentrations (µg/L; 0 – 2 m), June – September, 2012, 2013, and 2014.
The total dissolved phosphorus available to phytoplankton in Shadow Mountain
Reservoir and Grand Lake would be sufficient to support sustained growth of algae
(phytoplankton). As shown by previous studies (Morris and Lewis 1988), phytoplankton growth in these two lakes tends to be nitrogen limited rather than phosphorus limited (i.e., nitrogen is depleted before phosphorus). For the North Fork of the Colorado River and the two Grand Lake tributaries, total dissolved P was highest during snowmelt runoff and was moderately high at other times of high discharge. For the Granby Pump Canal, total dissolved P was highest in 24
September, near the end of the stratification season. Although Shadow Mountain Reservoir and
Grand Lake showed relatively little seasonal variation for dissolved P, concentrations of dissolved P tended to be higher during snowmelt and during fall than in August.
Particulate phosphorus often accounts for half or more of the total phosphorus in the water column of Grand Lake and Shadow Mountain Reservoir. This is typical for mountain lakes and reservoirs. A significant portion of the particulate phosphorus is tied up in living biomass, especially when phytoplankton biomass is high. Mineral particulate phosphorus tends to settle more rapidly than organic particulate P, although particles in the North Fork of the
Colorado River often are dominated by small particles that settle slowly. Initiation of pumping in mid June, 2012 was accompanied by a large increase in particulate phosphorus in Shadow
Mountain Reservoir and Grand Lake (Figure 10). The brief spike in particulate P observed for
Shadow Mountain Reservoir and Grand Lake in mid June suggests that initiation of pumping may have caused resuspension of sediment particles, but resuspended particles apparently were removed rapidly (e.g., through settling and export). Because of high snowmelt runoff in 2013 and 2014, concentrations of particulate P in Shadow Mountain Reservoir were higher in early
June, 2013 and 2014 than at the same time of year in 2012, and concentrations of particulate P generally were higher in 2013 and 2014 than in 2012. Particulate P in Shadow Mountain
Reservoir increased greatly in 2013 during the six week period with no pumping.
Concentrations of particulate P in Shadow Mountain Reservoir and Grand Lake were similar across dates in 2014 and were generally higher than concentrations in 2012 but not as high as during the period with no pumping in 2013.
Nitrogen compounds include ammonia and ammonium (total ammonia), nitrate (nitrite is present only at negligible concentrations in the lakes and their water sources), dissolved organic
25
nitrogen, and particulate nitrogen (which is principally organic; Table 3). Dissolved organic nitrogen and particulate nitrogen are not very useful for diagnosing the nutrient status of water with respect to the needs of algae. Organic N often is dominated by refractory molecules that do not nourish phytoplankton well. Therefore, the most meaningful interpretation of the nutrient status for phytoplankton with respect to nitrogen is based on total ammonia and nitrate (i.e., dissolved inorganic nitrogen).
Nitrate concentrations typically were higher in East Inlet and North Inlet than in the
North Fork of the Colorado River or the Granby Pump Canal (Figure 11). Nitrate concentrations in the Granby Pump Canal increased over the summer, with a peak concentration of about 50
µg/L as N in 2012 and peaks of ~80 – 100 µg/L in 2013 and 2014. Nitrate was nearly exhausted from the upper water column of Grand Lake and Shadow Mountain Reservoir over most of the growing season. Nitrate depletion in epilimnetic waters is explained by nitrogen demand of phytoplankton; nitrate demand of aquatic macrophytes and attached algae (e.g., attached to aquatic macrophytes in Shadow Mountain Reservoir) also can contribute significantly to nitrate depletion. Grand Lake and Shadow Mountain Reservoir probably experienced nitrogen deficiency for phytoplankton as indicated by severe depletion of nitrate from the growth zone.
There was some relief from nitrogen depletion in August and September, 2012 near the Shadow
Mountain dam, although most of the nitrate delivered by the Granby Pump Canal to the reservoir was removed (e.g., by phytoplankton uptake) before reaching Grand Lake. Nitrate was moderately high in Grand Lake during snowmelt runoff in 2013 and increased in Shadow
Mountain Reservoir and Grand Lake after the September storm in 2013. Nitrate concentrations in Shadow Mountain Reservoir were higher during snowmelt runoff in 2014 than in 2012 and
2013, but not quite as high as nitrate concentrations in the deep water of Grand Lake.
26
Figure 11. Concentrations of nitrate N in water sources and in surface water of Shadow Mountain Reservoir and Grand Lake.
27
Nitrogen in total ammonia (ammonia-N) was present at low concentrations (less than ~20
μg/L as N) for all stations except during August and October, 2013 for Shadow Mountain
Reservoir, and concentrations typically were <5 µg/L in surface waters of Shadow Mountain
Reservoir and Grand Lake. High concentrations of ammonia-N in Shadow Mountain Reservoir in 2013 (Table 3, Figure 12) were caused by release from organic matter within the reservoir
(Figure 7).
Overview of Physical and Chemical Characteristics: Water quality analysis indicates that Shadow Mountain Reservoir and Grand Lake have weakly buffered waters of low specific conductance, as is common in Colorado at high elevation, and that there can be sufficient algal growth during the growing season to produce high pH in the upper water column. Grand Lake was strongly affected by native water sources early in the growing season, especially in 2013 and
2014 when snowmelt runoff was high. After peak snowmelt runoff in 2012, introduction of water from Shadow Mountain Reservoir changed the characteristics of Grand Lake. Changes included higher specific conductances (dissolved solids content) and higher nutrient concentrations in surface water. The effect of pumped water on Grand Lake was less in 2013 and 2014 than in 2012, but Grand Lake still was strongly influenced by pumped flow at sometimes in 2013 and 2014 because effects of pumped flow can extend well beyond periods of pumping. Previous studies and recently observed concentrations of phosphorus and nitrogen suggest that phosphorus is not limiting in Grand Lake for phytoplankton but inorganic nitrogen depletion sometimes may cause nitrogen deficiency and limit growth rates for phytoplankton.
28
Figure 12. Concentrations of ammonia-N in water sources and in surface water of Shadow Mountain Reservoir and Grand Lake.
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Background Information on Transparency in Lakes
The optical properties of natural waters are determined by the combined optical properties of water molecules, dissolved substances, and suspended particles. Some optical properties also are affected by the incident light field. As light passes through even the purest water, its intensity is attenuated through absorption and scattering of photons across the visible spectrum. Photons are lost through absorption, but scattering (deflection) also is important because scattering increases the path length as light passes through water and thereby increases the probability of absorption. Transparency of water, which is the property of allowing transmission of light without absorption or scattering, and other optical properties can be quantified for a specific wavelength or a range of wavelengths (e.g., over the visible spectrum).
Secchi transparency is one of the simplest and most commonly used measures of transparency.
Attenuation coefficients calculated from the change in intensity with depth also provide information on transparency, and nephalometric measurements of turbidity provide information on the scattering properties of water.
Measurement of Transparency and Other Optical Properties of Water
Secchi Transparency: Secchi transparency (Secchi depth) is the depth within the water column to which a reflective disk remains visible to an observer at the surface. The standard disk for measurements of Secchi transparency is 8” (20 cm) in diameter and has alternating black and white quadrants, although other sizes and patterns of Secchi disks also are used. A viewing scope (view scope; an opaque tube with a transparent pane on one end) sometimes is used to reduce surface reflections when Secchi transparency is measured. Typically, use of a view scope increases measurements of Secchi transparency by about 5%, although the effect of the view
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scope depends on wind speed, accumulation of particles at or very near the surface, and other factors that affect optical properties of the surface layer. Secchi transparency is an apparent optical property (i.e., a property determined partly by the nature of the light field incident on the surface) and is affected by solar angle (time and season of measurement; Verschur 1997), cloud cover, visual acuity of the observer, and other factors. Partly because of its simplicity and widespread use, an interim numeric standard proposed for Grand Lake is based on Secchi transparency.
Attenuation coefficients: The attenuation of light within a water body follows a negative exponential function for any given wavelength, provided that the water body over the interval of measurement is uniform in concentration of substances that control absorption and scattering of light. The negative exponential curve can be represented as follows:
where irradiance (I) is photon flux or energy flux, z is depth, and the subscript t denotes total attenuation from all causes; K is the attenuation coefficient. If z is given in meters, the attenuation coefficient has units of m-1. For present purposes, quantum units are used for irradiance (μmol photons m-2 sec-1). Use of energy units (Watts m-2) gives slightly different coefficients but the difference is not meaningful for present purposes. The attenuation coefficient for pure water (Kw) varies with wavelength, and attenuation of long wavelengths (red light) is more rapid than attenuation of short wavelengths (blue light; Figures 13, 14). Thus, the spectral composition of light changes with depth in pure water (Figure 15) and also in natural waters.
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Figure 13. The visible spectrum, in relation to ultraviolet and infrared radiation. PAR (photosynthetically active radiation; 400 - 700 nm) covers most of the visible spectrum.
Figure 14. Attenuation coefficients over the visible spectrum and near ultraviolet (350 – 700 nm) for pure water (Morel and Maritorena 2001; left) and change in irradiance with depth for two wavelengths in pure water (right).
Figure 15. Typical spectral composition of surface irradiance for sunny and cloudy days (left; Bartlett et al. 1998 and NOAA Earth System Research Laboratory, Global Monitoring Division) and change with depth in the spectral composition of irradiance for pure water (right).
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Like Secchi transparency, K is an apparent optical property and is affected by both the optical medium and the nature of the incident light field (e.g., solar angle; Zheng et al, 2002).
Thus, measurements of K or Secchi transparency made at different times of day, in different seasons, or with different cloud cover may not be directly comparable. Unlike measurements of
Secchi transparency, however, attenuation coefficients can be measured over predefined depth intervals and for specific wavelengths (e.g., as in Figure 14). Attenuation coefficients can be measured in the field from profiles of integrated irradiance (i.e., over a range of wavelengths) with an underwater quantum sensor. Because attenuation varies with wavelength, attenuation coefficients measured in the field with a quantum sensor will vary with depth even if the water body over the interval of measurement is uniform in concentration of substances that control absorption and scattering of light.
Nephalometric turbidity can be measured in the field with an instrument that uses a detector at 90° to an infrared beam (typically 760±30 nm); the detector captures the portion of the beam that is scattered orthogonally by particles in the water. Turbidity measurements often are correlated with measurements of total suspended solids and other variables related to transparency, but not all particles scatter strongly in the near infrared wavelengths and scattering properties of particles vary with wavelength (e.g., scattering properties can differ between infrared and visible wavelengths).
Factors Affecting Transparency
When transparency is expressed as an attenuation coefficient (K) for a given wavelength,
K can be separated into additive components, as follows:
Kt = Kw + Kd + Ka + Kp
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where t denotes total, w denotes pure water, d denotes dissolved substances, a denotes algae, and p denotes non-algal particles. The problem of explaining transparency in any given natural water is to quantify each of the separate components that determine Kt, and the corresponding effects on transparency, for each wavelength of irradiance.
Wavelengths of solar radiation can be broken into three groups (Figure 13): ultraviolet
(<400 nm), PAR (photosynthetically available radiation; 400-700 nm), and infrared (>700 nm).
Visible light includes PAR and a narrow band (~380-400 nm) that is not included in PAR.
Ultraviolet radiation accounts for about 3% of surface irradiance (i.e., irradiance reaching the surface of the earth or of a natural body of water), PAR accounts for about 46%, and infrared radiation accounts for the balance. The exact partitioning depends on a number of factors, including elevation above sea level and atmospheric conditions (Figure 15).
Ultraviolet wavelengths receive attention in connection with potential damage to protoplasm, but are attenuated rapidly in most inland waters because they are absorbed selectively by water molecules and colored dissolved organic matter. Infrared wavelengths are absorbed rapidly by water and do not contribute significantly to irradiance below 2 m within the water column. Thus, the PAR spectrum dominates penetrating light. PAR also is the portion of the spectrum that accounts for photosynthesis and is visible to the human eye. For present purposes, the analysis focuses on the PAR range (400-700 nm), although attenuation spectra are available over the range of 350-700 nm.
At the surface of a lake, the amount of irradiance available across the PAR range is not constant; more photons are available at the medium and longer wavelengths than at the short wavelengths (Figure 15). In addition, the value of for each attenuation component varies across the wavelength spectrum. As PAR passes downward in the water column, its spectral
34
composition is changed by the additive influence of each of the components. Through modeling, the intensity and wavelength distribution of PAR can be computed at short intervals below the surface of a lake from the attenuation spectra and the concentrations of the substances that generate the spectra, as explained later in this report.
In addition to the effects of pure water, the components affecting transparency include dissolved substances, algae, and non-algal particulate matter. The effect of each of these components was estimated for Shadow Mountain Reservoir and Grand Lake during the 2012 –
2014 Particle Study.
Pure water: The effect of pure water is a physical constant; it does not vary from time to time or place to place (Figures 14 - 16). Pure water absorbs strongly in the ultraviolet wavelengths but also in the infrared and longer visible wavelengths (i.e., beyond ~600 nm). The wavelength of minimum attenuation for pure water is ~420 nm. Thus, most of the photons that remain at great depth in very clear waters correspond to the blue wavelengths (Figures 13 – 16).
Figure 16. Absorption spectra for water and substances in water. (Sources: A, Morel and Maritorena 2001; B, D, empirically derived for Grand Lake surface water, August 7, 2012; C, empirically derived for pure cultures of Anabaena planctonica and Asterionella formosa isolated from Shadow Mountain Reservoir and Grand Lake.
35
Dissolved Solids: Total dissolved solids include inorganic dissolved solids (mostly salts)
++ ++ + + - = - - that are common in inland waters (Ca , Mg , Na , K , HCO 3, SO 4, Cl , H4SiO4, NO3 ). Nitrate absorbs strongly in the ultraviolet wavelengths (~200 nm), but none of the salts have significant
PAR absorption. Therefore, the substances contributing to total ionic solids (as estimated by specific conductance) are irrelevant to transparency. Also included in total dissolved solids, however, is dissolved organic matter (DOM). It is typical to measure DOM in water as dissolved organic carbon (DOC, mg/L). DOM is approximately two times DOC. Thus, a DOC measurement of 5 mg/L corresponds approximately to 10 mg/L of DOM.
DOM can be divided into two parts: achromatic and chromatic. The achromatic compounds come primarily from the release of simple molecules from organisms as the organisms die or excrete metabolites. Examples include amino acids, simple sugars, and intermediary metabolic compounds. These compounds have no color within the PAR range; they do not contribute to the attenuation of light. The chromatic compounds are mostly of terrestrial origin and come into streams and lakes from their watersheds. Chromatic compounds form in soils as a result of the decomposition of complex carbohydrates that are derived from vascular plants. They absorb light selectively at the short wavelengths but only weakly at the longer wavelengths in the visible spectrum (Figure 16). Consequently, when they are abundant
(above 5-10 mg/L DOC), they impart a visible brown color to water. The chromatic effect of these compounds, which are typically referred to as humic and fulvic acids or simply as humic substances, is quite strong, even for relatively low concentrations (e.g., 3-4 mg/L DOC).
Typically, inland waters have a significant fraction (e.g., 50%) of chromatic compounds within the DOM pool. The chromatic fraction is increased where there is drainage from wetlands or
36
organic-rich soils. The transmission of humic compounds in mountainous terrain is accentuated by low hardness, which favors movement of humic substances.
For each water source on a given date, DOC concentration was determined by high temperature combustion followed by infrared gas analysis, and the attenuation spectrum for chromatic DOC was determined by spectrophotometry of filtered samples. Typically, DOC concentrations were near 3 - 5 mg/L in June, and concentrations declined slightly after snowmelt runoff (Figure 17). Concentrations in the Granby Pump Canal and in deep water for Grand Lake tended to be slightly higher than concentrations in Grand Lake’s surface waters or tributaries during the study. DOC concentrations in the Grand Lake tributaries and in the North Fork of the
Colorado River declined after snowmelt runoff and were slightly higher in 2013 and 2014 than in
2012. Shadow Mountain Reservoir and surface waters in Grand Lake had similar DOC concentrations in 2012, but Shadow Mountain Reservoir had higher DOC concentrations in 2013 when pumping was stopped. Relatively high concentrations of DOC in the Granby Pump Canal and in deep water for Grand Lake reflect high DOC concentrations during snowmelt runoff, and the high concentrations in Shadow Mountain Reservoir in 2013 were the result of high rates of photosynthesis during the six-week period with no pumping. The seasonal changes in DOC concentrations in Shadow Mountain Reservoir and surface water of Grand Lake reflect the seasonal changes in DOC concentrations for the tributaries.
37
Figure 17. Concentrations of dissolved organic carbon in water sources and in surface water of Shadow Mountain Reservoir and Grand Lake.
38
Attenuation spectra varied somewhat across samples, but the spectrum shown in Figure
16 (panel B) is typical. Short (blue) wavelengths are strongly attenuated by chromatic DOC. The longest visible wavelengths (red wavelengths) are only weakly attenuated by chromatic DOC.
Attenuation coefficients at 350 nm give an indication of the contribution of dissolved substances to total attenuation and how this component of total attenuation varies across stations and across dates (Figure 18). The seasonal pattern for Kd at 350 nm was generally similar to the seasonal pattern for DOC concentrations. Kd, 350 nm decreased from June through October for tributaries and for surface waters, particularly in 2013 and 2014; as for DOC concentration, there was a spike in Kd after the September storm in 2013. During the 2013 stop-pump period when DOC concentrations were high in Shadow Mountain Reservoir, however, there was not a corresponding increase in Kd. The increase in DOC during the stop-pump period was caused partly by release of achromatic compounds by phytoplankton.
39
Figure 18. Attenuation coefficient at 350 nm, m-1.
40
Phytoplankton: The abundance of phytoplankton can be estimated from chlorophyll, which typically constitutes 0.5 - 2% of the dry mass of algae. Algal absorbance, including chlorophyll plus the algal non-chlorophyll components, has peaks at both high and low wavelengths (Figure 16, panel C). Because the wavelength of minimum attenuation for phytoplankton cells is near 530 nm (green light), high abundance of phytoplankton gives water a greenish color.
Attenuation spectra as a function of chlorophyll concentration are available from the literature for marine phytoplankton (e.g., Morel and Maritorena 2001), but similar relationships also can be derived for pure cultures of phytoplankton. The relationship between chlorophyll concentration and attenuation spectrum, as shown in Figure 16, was derived from attenuation spectra for algal particles (i.e., the difference between spectra for unfiltered and filtered samples) in cultures isolated from Shadow Mountain Reservoir and Grand Lake. The shape of the attenuation spectrum for suspended algae (phytoplankton) changes slightly with abundance of phytoplankton (chlorophyll concentration; Figure 16). The spectra for algal absorption vary according to the size distribution, taxonomic composition, and growth conditions of the phytoplankton community. For the present study, a single set of attenuation spectra for phytoplankton was used for modeling (i.e., spectra were determined by measured chlorophyll concentrations and the relationships shown in Figure 16).
The Granby Pump Canal transported very little chlorophyll, as would be expected for the water from the hypolimnion of Granby Reservoir (Figure 19). Suspended and attached algae can grow within the Pump Canal when the water is stagnant. However, algal biomass that accumulates in the Pump Canal affects transport of chlorophyll to Shadow Mountain Reservoir only for brief periods of time. The North Fork of the Colorado River and the tributaries of Grand
41
Figure 19. Concentrations of chlorophyll a.
42
Lake also had low concentrations of chlorophyll. Chlorophyll transported by mountain streams in Colorado is derived almost entirely from entrainment of periphyton (attached algae). In 2012, peak concentrations of chlorophyll in Shadow Mountain Reservoir and Grand Lake occurred in
June and were higher than would be typical for montane lakes in a natural state (< 5 μg/L), but did not reach nuisance levels. Chlorophyll concentrations in the lakes decreased abruptly after pumping resumed and increased again in August. In 2013, snowmelt runoff was higher than in
2012 and chlorophyll concentrations in the lakes were near 2 µg/L in June. After pumping was stopped in late July, chlorophyll concentrations in Shadow Mountain Reservoir rose sharply, especially near the dam. Chlorophyll concentrations near the dam were ~30 µg/L through late
July and August. Chlorophyll concentrations also increased in Grand Lake during the period with no pumping, but only slightly. Chlorophyll concentrations in Shadow Mountain Reservoir were much lower after the storm in September, 2013. Chlorophyll concentrations decreased in
Grand Lake in August, 2013, shortly before pumping resumed, probably as a result of increased mixing depth. In 2014, chlorophyll concentrations in both lakes were low (<2 µg/L) during snowmelt runoff and increased to ~5-10 µg/L in early September. The relatively high concentrations of chlorophyll in September 2014 are explained by high residence time.
Suspended Solids: In addition to measurements of chlorophyll, which provide information on algal particles, suspended solids were measured as total suspended solids (TSS) and ash-free dry mass (AFDM). Total suspended solids were measured gravimetrically for water taken at specific depths. Suspended solids consist of two fractions: organic and inorganic. Ash- free dry mass is a measurement of the organic fraction of suspended solids; the difference between TSS and AFDM gives inorganic solids. The organic fraction consists of two subfractions: algal and non-algal. The mass of algal particles can be estimated from
43
measurements of chlorophyll and an assumed ratio of chlorophyll to algal biomass (see below).
Thus, the measurements at hand allow estimates of total suspended solids, mass of inorganic particulate matter, mass of phytoplankton, and mass of non-algal organic particulate matter.
Concentrations of total suspended solids generally were low (~1 mg/L) in North Inlet and
East Inlet, except during snowmelt runoff and briefly after the storm in September, 2013 (~5 mg/L; Figure 20). In comparison with the Grand Lake tributaries, concentrations in the North
Fork of the Colorado River were slightly higher on most dates (~1.5 mg/L) and much higher
(~25 mg/L) at times of the highest discharge. TSS concentrations in the Granby Pump Canal usually were slightly higher than those of other water sources (~2 – 3 mg/L), but much lower than the highest concentrations observed in the North Fork of the Colorado River. Because the two stations on the Granby Pump Canal that were sampled in 2012 had similar concentrations of
TSS, only one station was sampled subsequently. In 2012, both Shadow Mountain Reservoir and Grand Lake showed a spike in TSS that coincided with the August, 2012 spike in TSS for the North Fork of the Colorado River. High TSS in Shadow Mountain Reservoir in 2013 reflected high phytoplankton biomass after pumping was stopped.
44
Figure 20. Concentrations of total suspended solids.
45
Ash-free dry mass (organic particles) is calculated from loss on ignition, and the concentration of inorganic particles is calculated from measurements of TSS and AFDM. Ash- free dry mass typically accounted for about half of total suspended solids on most dates in flowing waters except for the North Fork of the Colorado River and slightly more than half of
TSS in Shadow Mountain Reservoir and Grand Lake (Figures 20 – 21). At high flows, inorganic particles accounted for most of the suspended solids in the North Fork of the Colorado
River (Figures 20 – 22).
The concentration of non-algal organic particles for a sample is calculated as the difference between AFDM (all organic particles) and algal particles. The concentration of algal particles is calculated from chlorophyll and an assumed ratio of algal biomass to chlorophyll; the
AFDM:Chl ratio varies across algal taxa and according to growing conditions but can be determined empirically (i.e., for samples that do not contain other, non-algal organic particles).
Samples of particles from Shadow Mountain Reservoir and Grand Lake were centrifuged through colloidal silica in order to separate algal particles from non-algal particles. AFDM:Chl ratios for purified samples were near 160 or higher. Centrifugation through colloidal silica does not remove all of the non-algal particles from the purified samples, but the purified samples with the lowest AFDM:Chla ratios are assumed to be the most pure. The relationship between AFDM and chlorophyll a for samples from Shadow Mountain Reservoir and Grand Lake (Figure 23) suggests that 160 is a reasonable estimate of the AFDM:Chl ratio for algal particles in the two lakes (i.e., chlorophyll a is near 0.63% of algal biomass). During the stop-pump period in 2013, when chlorophyll a in Shadow Mountain Reservoir exceeded 20 µg/L, most of the organic particles in Shadow Mountain Reservoir were algal particles. Deep water in Grand Lake had a much higher AFDM:Chl ratio, indicating significant amounts of non-algal organic matter.
46
Figure 21. Concentrations of ash-free dry mass.
47
Figure 22. Concentrations of inorganic particles.
48
Figure 23. Ash-free dry mass versus chlorophyll a for particulate matter in Shadow Mountain Reservoir and Grand Lake. The dashed line indicates a ratio of AFDM:Chl a of 160 (i.e., chlorophyll a is ~0.63% of algal organic matter).
Concentrations of algal particles were estimated from chlorophyll a and an assumed
AFDM:Chl ratio of 160, and concentrations of non-algal organic particles were calculated as the difference between AFDM and algal particles. Nearly all of the organic particles in flowing waters were non-algal particles (Figures 21, 24). The North Fork of the Colorado River carried high concentrations of non-algal organic particles at times of high flow, although most of the particles carried by the North Fork were inorganic (Figures 22, 24). For surface waters in the lakes, non-algal particles often accounted for at least half of organic particles. Some of the non- algal organic particles in the lakes were derived from watershed sources. However, weak correlation between non-algal organic particles (Figure 24) and chlorophyll (Figure 19) for
Shadow Mountain Reservoir during the 2013 period when pumping was stopped suggests that the non-algal organic particles may include significant amounts of algal detritus, heterotrophs dependent on phytoplankton, or other non-algal organic particles that are ultimately derived from algae. Uncertainty in the AFDM:Chl ratio that is used for calculations of algal and non-algal organic particles also may partly explain the correlation between algal and non-algal organic particles. 49
Figure 24. Non-algal organic particles.
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The Transparency Record
The record of transparency for Shadow Mountain Reservoir and Grand Lake for the
Particle Study includes measurements of Secchi transparency, attenuation coefficients for PAR, and turbidity. Because the human eye is sensitive to PAR, Secchi transparency is related to the attenuation coefficient for PAR (t). The two are not the same, however; scattering, which is related to turbidity, can reduce the Secchi depth more quickly than it reduces the extinction coefficient for PAR. The Secchi depth is relevant to the general appearance of a lake for recreational or aesthetic purposes. The attenuation coefficient is particularly valuable for analytical purposes because it is a quantitative measurement of transparency for predefined depth intervals (e.g., 0 – 4 m) and can be divided into components of absorbance that can be attributed to specific water quality variables.
Secchi transparency can be measured only for waters that are deeper than the Secchi depth, and measurements are difficult to obtain in flowing water. Therefore, Secchi depth measurements are available only for Shadow Mountain Reservoir and Grand Lake, and not flowing waters. Also, measurements for the Shadow Mountain connecting channel are not available for some dates when Secchi depth was greater than the depth of the water and Secchi depth could not be measured precisely. Similarly, field estimates of PAR attenuation coefficients can be obtained only when irradiance measurements can be made over a range of depths. Thus, field based measurements of PAR attenuation also are available only for the two lakes. Attenuation coefficients and Secchi depths can be calculated for flowing waters when absorbance components are known, however. Measurements of turbidity, which are made with a handheld meter at specific points within the water column, are available for both lakes and flowing waters.
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Secchi Transparency
Secchi transparency in Shadow Mountain Reservoir was measured with and without use of a view scope. Use of the view scope had a significant effect on measurements of Secchi transparency, but the effect was small in comparison with the range of Secchi transparency for the lakes (Figure 25). Measurements of Secchi transparency made with the view scope tend to be about 5% greater than measurements without the view scope; for Secchi depths less than about 2 m, the effect of the view scope usually is very small. Measurements of Secchi transparency shown in Figure 26 were made with a view scope, but measurements made without a view scope show similar patterns of spatial and temporal variation.
Figure 25. Effect of view scope on measurements of Secchi transparency.
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Figure 26. Secchi transparency for stations on Shadow Mountain Reservoir and Grand Lake.
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At the end of the period when pumping was stopped for maintenance in June, 2012,
Secchi transparency was near 2 m in Shadow Mountain reservoir, except at SM-NOR2, where
Secchi transparency was about 3 m; at the same time, Secchi transparency in Grand Lake was near 3 m (Figure 26). After pumping resumed in mid June, Secchi transparency declined briefly at SM-NOR2 but increased at other stations. Secchi transparency in early July was near 4 m at
SM-DAM and was near 3.5 m at other stations. Secchi transparency in early August was lower at all stations than in July, and Secchi transparency remained stable in Shadow Mountain
Reservoir and increased steadily in Grand Lake through September and October. Over most of
June and July in 2013, Secchi transparency in Shadow Mountain Reservoir was near 2.5 m;
Secchi transparency decreased to ~1 m after pumping was stopped in late July but was near 3 m in early October. Secchi transparency at GL-MID was over 4 m in early June and declined to about 2 m by the time pumping was stopped in late July. Secchi transparency at GL-MID changed little over the six-week period with no pumping but was > 4 m in early October.
Transparency at GL-CHL was similar to transparency in Shadow Mountain Reservoir in June and early July, but there was little difference in transparency between GL-CHL and GL-MID during August. In 2014, Secchi transparency in Shadow Mountain Reservoir was near 2m in early June and mid July. In August and September, Secchi transparency was a little over 3 m in mid August and early September near the dam in Shadow Mountain Reservoir and closer to 2 m at the other two stations. Secchi transparency in Grand Lake in 2014 fell within the range of ~3-
4 m on the dates of routine sampling. Because sampling in 2014 was less frequent than in 2012 or 2013, sampling did not capture the full range of variation that occurred in 2014. Data collected by GCWIN, which are presented in a later section of the report, provide additional information on Secchi transparency for both Shadow Mountain and Grand Lake.
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Spatial variation in Secchi transparency was greater for Shadow Mountain Reservoir than for Grand Lake as shown by the mapping studies in 2012 and 2014 (Figure 27). At the time of the first mapping study, Secchi transparency was less than ~2.5 m across Shadow Mountain
Reservoir and greater than ~2.5 m in Grand Lake. The lowest values of Secchi transparency in
Shadow Mountain Reservoir were between the mouth of the North Fork and the connecting channel. In August, 2012, the lowest values of Secchi transparency were for Shadow Mountain
Reservoir, near the mouth of the North Fork of the Colorado River; the areas of highest transparency also were in Shadow Mountain Reservoir, near the outlet of the Granby Pump
Canal. The spatial pattern of Secchi transparency for Shadow Mountain Reservoir suggested movement of water at that time from near the mouth of the North Fork of the Colorado River north, along the east shore. At the time of the third mapping study, the areas of lowest transparency in Shadow Mountain Reservoir were near the mouth of the North Fork of the
Colorado River and near the connecting channel. Transparency was moderately low between the mouth of the pump canal and the North Fork delta.
55
Figure 27. Spatial variation in Secchi transparency for Shadow Mountain Reservoir and Grand Lake at the times of the three mapping studies.
Attenuation Coefficients
When pumping was stopped for maintenance in June, 2012, Kt for PAR (0 – 2 m) was near 0.8/m for the stations on Grand Lake and for SM-DAM and SM-NW1, but Kt was slightly lower (greater transparency) for the other stations on Shadow Mountain Reservoir (Figure 28).
After pumping resumed in mid June, Kt dropped at SM-DAM and at SM-NW1 and also for the
56
Figure 28. Diffuse attenuation coefficient for PAR (0 – 2 m) for stations on Shadow Mountain Reservoir and Grand Lake.
57
stations on Grand Lake. In July and September, Kt increased from south to north in Shadow
Mountain Reservoir. For most stations on Shadow Mountain Reservoir, Kt was highest in
August. Kt at GL-MID declined slightly from July to September, but did not fall much below
0.7/m. In 2013, Kt for PAR in Shadow Mountain Reservoir was less than 1.0/m in early June but was ~2.0/m in late July and August when pumping was suspended. Kt for PAR was near 0.2/m at GL-MID in early June and ranged from about 0.5 – 1.0/m afterward. Kt was similar across sampling dates in 2014, with slightly higher values for stations on Grand Lake than for stations on Shadow Mountain Reservoir.
Attenuation coefficients for PAR were mapped across Shadow Mountain Reservoir and
Grand Lake as part of the mapping studies in 2012 and 2014 (Figure 29). Attenuation coefficients in Shadow Mountain Reservoir were high (low transparency) near the mouth of the
North Fork of the Colorado River and in shallow water near the connecting channel. Moderately high attenuation coefficients were measured in Grand Lake near the mouth of East Inlet and
North Inlet at the time of the first mapping study, but these areas of high attenuation were very localized. In July, 2014, attenuation coefficients over 2-4 m generally were lower than over 0-
2m near SM-MID
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Figure 29. Spatial variation in diffuse attenuation coefficients for PAR (0 – 2 m, except 0 - 2 m and 0 - 4 m for July 29, 2014) in Shadow Mountain Reservoir and Grand Lake at the times of the three mapping studies.
Turbidity
Turbidity was measured for flowing waters and for the lakes (Figure 30). Turbidity in the native tributaries increased during times of high discharge but generally was low from July through September. Turbidity in the North Fork of the Colorado River increased in August,
2012 after a spike in discharge, and turbidity in the North Fork of the Colorado River was
59
Figure 30. Turbidity in flowing waters and in surface water in Shadow Mountain Reservoir and Grand Lake.
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particularly high during snowmelt runoff in 2013. Turbidity in the Granby Pump Canal was moderately high just after pumping resumed in June and then declined to intermediate values.
High turbidity in the Granby Pump Canal in June may have been caused by resuspension of material deposited in the canal or by high turbidity of waters entering Granby Reservoir during snowmelt runoff. Turbidity in Shadow Mountain Reservoir and Grand Lake followed the same general pattern as turbidity in the tributaries in 2012. Seasonal changes in turbidity for Shadow
Mountain Reservoir in 2013 were dominated by a large increase in turbidity during the period when pumping was suspended, followed by a drop to much lower values in early October. As in
2013, turbidity in the North Fork of the Colorado River was high during snowmelt runoff in
2014 and then declined to values only slightly higher than values for the Grand Lake tributaries.
Turbidity was high in Shadow Mountain Reservoir in 2014, but not nearly as high as during the six-week period with no pumping in 2013, and turbidity in both lakes increased slightly toward the end of the growing season in 2014.
Turbidity varied widely across Shadow Mountain Reservoir and Grand Lake on May 31
– June 1, 2012, when the highest turbidity values near the surface were in a region between the mouth of the North Fork of the Colorado River and the connecting channel (Figure 31). In
August, 2012, and July, 2014, turbidity also was high near the mouth of the North Fork of the
Colorado River, but high turbidity on those dates did not extend beyond the southern half of
Shadow Mountain Reservoir, except in shallow areas near the connecting channel and
Columbine Creek.
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Figure 31. Spatial variation in turbidity near the surface of Shadow Mountain Reservoir and Grand Lake at the times of the three mapping studies (0-2 m, except 0-2 m and 0-4 m for July 29, 2014).
One purpose of the July, 2014 mapping study was to provide information on the fate of particles from the Granby Pump Canal. Although hydrologic conditions throughout 2014 were not ideal for this purpose, the 2014 mapping study did provide useful information on spatial variation in turbidity across multiple depths. Between the mouth of the Granby Pump Canal and the North Fork delta, and especially northeast of the North Fork delta, turbidity was higher at a depth of 2 m than at the surface (Figure 31, lower panels). These observations are consistent with
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subsurface flow of water from the pump canal northward along the west bank of Shadow
Mountain Reservoir, but the highest turbidity values at 2 m also may reflect the influence of particles carried by the North Fork of the Colorado.
Separation of Transparency Components
The effects of individual components of transparency can be computed from the attenuation spectra for pure water, chromatic dissolved organic compounds, phytoplankton, and non-algal particulate material. The effects of pure water are consistent across samples (Figure
16), but the spectra for the other components were determined empirically for individual samples.
Dissolved Organic Matter
For each water source on a given date, the attenuation spectrum for chromatic DOC was determined by spectrophotometry of filtered samples. Attenuation spectra varied somewhat across samples, but the spectrum shown in Figure 16 is typical. Short (blue) wavelengths are strongly attenuated by chromatic DOC, but the longest visible wavelengths (red wavelengths) are only weakly attenuated by chromatic DOC.
Phytoplankton
The attenuation spectrum for suspended algae (phytoplankton) retains nearly the same shape for varying abundances of algae, but the attenuation coefficient for each wavelength is adjusted for the quantity of algae, measured as chlorophyll a (Figure 16). The attenuation spectra shown in Figure 16 were derived from measurements of attenuation spectra for pure
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cultures of algae isolated from Shadow Mountain Reservoir and Grand Lake, after subtraction of attenuation spectra for dissolved substances. The attenuation spectra for algae vary according to algal abundance, species composition, and size distribution of algal cells, but it was not feasible to determine relationships between chlorophyll concentration and attenuation spectra for phytoplankton for individual samples.
Non-Algal Particles
Attenuation spectra for non-algal particles were determined by subtracting the attenuation spectra for algal particles from the measured attenuation spectra for all particles (i.e., spectra for unfiltered samples, corrected for attenuation by dissolved substances).
Spectral Interactions
Because attenuation over the PAR spectrum is different for individual components, the wavelength distribution of light changes at progressive depths below the surface. Pure water, which absorbs selectively at longer wavelengths, shifts the spectrum of downwelling irradiance toward violet and blue wavelengths, whereas dissolved organic matter and non-algal particles have the opposite effect and shift the spectrum toward red wavelengths. Algae, which absorb selectively at long and short wavelengths, shift the downwelling irradiance toward the middle
(green) portion of the spectrum. The mixture of these effects was computed progressively at 2 cm intervals for the water columns of Grand Lake and Shadow Mountain Reservoir from the spectral information on individual components and empirical data on the abundance of each component.
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Relative contributions of individual components to attenuation at any given wavelength or across the entire range of wavelengths change with depth. At greater depths, the more readily absorbed wavelengths become scarce and the most penetrating portions of the spectrum predominate. Therefore, the spectrum at one depth will differ from the spectrum below it, and Kt for PAR typically will decline with depth. Kt for PAR may increase with depth, however, if the concentration of particles or DOC increases with depth. Figure 32 shows an example from the middle of Grand Lake. The relative importance of DOC and non-algal particles changes somewhat with depth as the spectrum changes, but these changes are fairly consistent across stations. For present purposes, the analysis will be focused on the upper 4 m of the water column.
Figure 32. Attenuation spectra for components of transparency (left) and spectral change with depth in Grand Lake (right) for Grand Lake (GL-MID) on August 7, 2012. The wavelength of minimum attenuation was ~580 nm; 23% of surface irradiance, as PAR, remained at 2 m and 6.6% of surface irradiance remained at 4 m.
Attenuation of PAR irradiance in waters of Shadow Mountain Reservoir and Grand Lake was dominated by non-algal particles, except in Shadow Mountain Reservoir in 2013 (Figures 33
- 34). Typically, non-algal particles (organic and inorganic) accounted for ~40 - 60% of the total attenuation of PAR (Tables 4 - 6, Figure 34). However, when pumping was stopped in 2013,
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attenuation near the dam and the mid station in Shadow Mountain Reservoir was strongly influenced by algal particles. The relative effect of dissolved organic matter on transparency was slightly higher in 2014 than in 2012, and for Grand Lake, the relative effect of non-algal particles was slightly lower in 2014 than in 2012. Results of modeling are summarized in Figure 35.
Figure 33. Components of attenuation, expressed as K, for PAR (0 – 4 m) for stations on Shadow Mountain Reservoir. Kw is attenuation by water, Kd is attenuation by dissolved substances, Ka is attenuation by algal particles, and Kp is attenuation by non-algal particles.
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Figure 34. Components of absorbance, expressed as K, for PAR (0 – 4 m,) for stations on Grand Lake. Kw is attenuation by water, Kd is attenuation by dissolved substances, Ka is attenuation by algal particles, and Kp is attenuation by non-algal particles.
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Attenuation Component Jun 14 Jun 20 Jul 10 Aug 7 Sep 6 Oct 29 SM-DAM Dissolved substances 12.1 18.3 18.8 13.3 12.7 --- Algal particles 22.0 5.8 15.0 17.5 8.6 --- Non-algal particles 45.3 41.6 38.9 49.4 53.8 --- SM-MID Dissolved substances 11.7 13.1 19.5 17.0 13.3 --- Algal particles 15.7 11.6 13.3 13.9 12.4 --- Non-algal particles 45.1 51.1 41.9 50.9 52.2 --- SM-NW1 Dissolved substances 10.2 14.0 19.0 14.4 10.2 --- Algal particles 18.4 8.5 13.3 15.2 12.1 --- Non-algal particles 50.2 46.4 42.2 50.5 56.7 --- SM-NOR2 Dissolved substances 11.7 10.5 15.2 13.1 8.3 6.6 Algal particles 8.2 9.2 12.3 12.1 11.3 7.5 Non-algal particles 52.9 56.5 48.2 53.8 61.7 67.7 GL-CHL Dissolved substances 24.1 13.8 18.6 11.8 11.1 --- Algal particles 25.7 9.5 11.7 14.8 10.1 --- Non-algal particles 25.2 53.5 45.9 51.7 57.9 --- GL-WES Dissolved substances 17.1 15.7 20.7 14.3 12.1 --- Algal particles 15.3 10.8 14.7 19.8 14.7 --- Non-algal particles 44.8 46.6 40.8 43.5 51.5 --- GL-MID Dissolved substances 25.0 17.7 24.1 15.1 10.8 13.6 Algal particles 15.6 11.1 17.2 18.8 13.5 6.6 Non-algal particles 38.0 44.4 36.2 44.7 53.1 60.1
Table 4. Percent contribution to PAR attenuation (0 – 4 m) by dissolved substances, algal particles, and non-algal particles in 2012.
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Attenuation Component Jun 10 Jul 1 Jul 17 Jul 31 Aug 8 Aug 29 Oct 8 SM-DAM Dissolved substances 23.3 24.7 19.7 11.8 9.8 7.5 19.9 Algal particles 5.9 10.9 7.3 41.7 41.2 42.1 10 Non-algal particles 45.8 43.7 49 32.2 32.3 38.9 48.4 SM-MID Dissolved substances 23.2 26.3 16 11.9 9.8 7 22.8 Algal particles 7.8 9.6 9.8 34.7 39.8 37.7 9.8 Non-algal particles 48.3 41.6 52.8 36.9 38.3 42.8 44.8 SM-NW1 Dissolved substances 27.2 22.6 16.1 11 10.1 10.2 23.3 Algal particles 5.7 10.3 9.1 22.9 39.8 16.8 8.2 Non-algal particles 32 47.5 53.7 50.2 38.5 57.5 46.8 GL-CHL Dissolved substances 33.7 27 17.1 14.8 16.9 21.9 38.4 Algal particles 6.9 11.1 9.2 9.1 9.3 11.2 12 Non-algal particles 29.3 36.2 49.2 48.7 54.3 48 28.4 GL-MID Dissolved substances 15.9 29.5 21.4 16.4 20.7 23 37.6 Algal particles 2.1 7.6 9.7 9.2 14.7 11.9 14.9 Non-algal particles 12.5 27.1 47.8 55.6 42.1 37.1 27.6
Table 5. Percent contribution to PAR attenuation (0 – 4 m) by dissolved substances, algal particles, and non-algal particles in 2013.
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Attenuation Component Jun 10 Jul 30 Aug 25 Sep 8 SM-DAM Dissolved substances 24.1 16.1 14.6 19.2 Algal particles 5.1 8.0 14.6 16.3 Non-algal particles 49.9 55.7 51.4 43.7 SM-MID Dissolved substances 24.1 17.8 12.8 14.3 Algal particles 5.5 6.2 5.7 22.4 Non-algal particles 49.3 47.9 63.0 42.7 SM-NW1 Dissolved substances 29.5 16.6 10.8 14.4 Algal particles 7.0 9.3 10.7 23.9 Non-algal particles 42.1 53.1 61.8 43.9 GL-CHL Dissolved substances 41.1 20.9 27.3 18.3 Algal particles 4.6 3.5 18.7 12.8 Non-algal particles 21.8 49.9 31.9 39.8 GL-MID Dissolved substances 42.0 23.8 23.7 15.9 Algal particles 11.2 11.5 11.9 17.2 Non-algal particles 23.8 39.6 39.9 44.5
Table 6. Percent contribution to PAR attenuation (0 – 4 m) by dissolved substances, algal particles, and non-algal particles in 2014.
Figure 35. Ranges and quartiles of % PAR attenuation (0 – 4 m) by dissolved substances, algal particles, and non-algal particles for Shadow Mountain Reservoir (left) and Grand Lake (right).
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Direct comparison of modeling results for the present study and the 2009 study
(McCutchan 2010) is not possible because of differences in the analytical methods. Attenuation spectra for particles were measured directly for samples collected in 2012 - 2014 but not for samples collected in 2009, and the assumed shape of the attenuation spectra for non-algal particles differed from the measured spectra for non-algal particles collected as part of this study.
Also, the period of record for the 2009 study was much shorter than the period of record for the present study, which covered a wider range of hydrologic conditions.
Regression Analysis of Variation in Transparency
An alternate way of summarizing controls on variability of transparency for Shadow
Mountain Reservoir and Grand Lake involves regression analysis. Secchi depth is used as the dependent variable (i.e., the responding variable) and each of the measured components of absorbance can be used as an independent variable (causal variable). The attenuation coefficient also could be used as the dependent variable, but the interim numeric clarity standard for Grand
Lake is based on Secchi transparency. Modeling of light attenuation over the PAR spectrum
(previous section) was based on attenuation spectra for pure water, dissolved substances, phytoplankton, and non-algal particles. Inorganic particles and other non-algal particles were not treated separately. With regression analysis, however, it is possible to treat inorganic particles and non-algal organic particles as separate, independent variables.
The regression procedure operates in step-wise fashion. It selects first the variable that provides the greatest explanation of variability. It then searches for additional explanation beyond what was explained by the first variable, and progresses step-wise to the point at which there is no further significant explanation of variability. If the distributions of independent
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variables considered here are normalized by log transformation, linearization is accomplished by log-transforming the dependent variable. Concentrations of algal particles are weakly but significantly correlated with concentrations of non-algal organic particles (log transformed, p <
0.0001, r2 = 0.28; Figure 36), but other independent variables do not show significant correlations. The weak correlation between algal particles and non-algal organic particles may reflect temporal correlation of processes that generate particles (e.g., production of algal particles and macrophyte particles is highest during summer) or resuspension of particles (i.e., because resuspended material includes particles of multiple origins); alternatively, this correlation may reflect variation in the AFDM:Chl ratio, which affects calculations of algal particles and non- algal organic particles from measurements of AFDM and chlorophyll a.
Figure 36. Correlation matrix for independent variables (log transformed) used in stepwise regression.
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For Secchi transparency (log transformed), 55% of the variance is explained by algal particles (Table 6). Although it is important to acknowledge the correlation between algal particles and non-algal organic particles, which may affect apportioning of variance among the independent variables, the colinearity between these variables is not strong and some additional variance can be explained if the concentration of non-algal organic particles is included as an independent variable (Table 7). With three independent variables (algal particles, non-algal organic particles, inorganic particles), 68% of the variance is explained, which is similar to the fraction of variance explained by total suspended solids (73%). Virtually all of the variability among sites and across dates can be attributed to particles. Dissolved substances (Kd, 350 nm) did not have a significant additional effect on Secchi transparency. Because concentrations of colored dissolved organic matter and inorganic particles were similar across stations and over time, the effects of dissolved substances on transparency does not vary greatly from one place to another or one time to another. The results of the regression analysis do not indicate that dissolved substances are unimportant, but rather that they do not help to explain much of the variance in transparency for this data set, after other variables are considered. The estimated regression coefficients in Table 7 suggest that, for a given mass of particles, algal particles have a slightly greater effect on transparency than non-algal organic particles or inorganic particles.
A second approach to regression analysis is based on prediction of inverse Secchi transparency, which is linearly related to particle concentrations (untransformed; Table 8).
Without log transformation, the proportion of variance attributed to a given independent variable may be overestimated. However, prediction of inverse Secchi transparency from linear combinations of the independent variables makes it possible to estimate the proportional contribution of each independent variable to the dependent variable (inverse Secchi
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transparency; see below). Results of these analyses support the conclusions that particles account for almost all of the variation in transparency and algal particles account for the largest proportion of variance among the independent variables. For a given concentration of particles, algal particles and non-algal organic particles have nearly equal effects on transparency, and the effect of inorganic particles is about half the effect of organic particles.
Parameter Estimate (±SE) Sequential R2 Significance Intercept 0.68 (±0.024) --- p < 0.0001 Total suspended solids, mg/L -0.70 (±0.047) 0.73 p < 0.0001
Intercept 0.31 (±0.012) --- p < 0.0001 Algal particles, mg/L -0.28 (±0.039) 0.55 p < 0.0001 Non-algal organic particles, mg/L -0.20 (±0.055) 0.62 p < 0.0005 Inorganic particles, mg/L -0.13 (±0.037) 0.68 p < 0.001
Kd (350 nm), /m ------NS
Table 7. Results of regression and stepwise multiple regression of Secchi transparency (Zsd), applied to samples collected in 2012 - 2014; chlorophyll a is assumed to be 0.63% of algal particles. Units of Zsd are m. All variables are log-transformed (log10).
Parameter Estimate (±SE) Sequential R2 Significance Intercept 0.12 (±0.019) --- p < 0.0001 Total suspended solids, mg/L 0.11 (±0.005) 0.84 p < 0.0001
Intercept 0.15 (±0.028) --- p < 0.0001 Algal particles, mg/L 0.11 (±0.010) 0.76 p < 0.0001 Non-algal organic particles, mg/L 0.12 (±0.021) 0.81 p < 0.0001 Inorganic particles, mg/L 0.08 (±0.017) 0.85 p < 0.0001
Kd (350 nm), /m ------NS
Table 8. Results of regression and stepwise multiple regression of inverse Secchi transparency (1/Zsd), applied to samples collected in 2012 and 2013; chlorophyll a is assumed to be 0.63% of -1 algal particles. Units of 1/Zsd are m . Independent variables are untransformed.
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Results of the regression analysis shown in Table 8 make it possible to estimate the proportional contribution of each independent variable to transparency (i.e., inverse Secchi transparency; Tables 9 – 11, Figure 37) for individual water samples. These results are generally similar to results of modeling shown in Tables 4 – 6, but it is possible with regression analysis to differentiate between effects of non-algal organic particles and inorganic particles. Algal particles had relatively little effect on transparency at most times, but algal particles were the dominant factor affecting transparency in Shadow Mountain Reservoir during the period of no pumping in 2013; otherwise, non-algal organic particles typically were the dominant factor affecting transparency. Even though the effect of inorganic particles on transparency was weaker than the effect of organic particles, when normalized for concentration (Table 8), inorganic particles had an important effect on transparency. The effect of inorganic particles was greatest during snowmelt runoff and after other periods of high discharge (e.g., in August, 2012).
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Attenuation Component Jun 14 Jun 20 Jul 10 Aug 7 Sep 6 Oct 29 SM-DAM Algal particles 35.4 8.1 19.6 18.7 11.9 --- Non-algal organic part. 12.9 25.1 17.3 22.1 20.8 --- Inorganic particles 17.9 26.0 15.2 28.7 34.5 --- SM-MID Algal particles 28.3 19.7 15.4 15.9 15.8 --- Non-algal organic part. 21.4 24.1 24.8 32.3 23.6 --- Inorganic particles 20.7 18.0 20.3 25.9 28.4 --- SM-NW1 Algal particles 30.2 16.1 15.8 19.4 18.0 --- Non-algal organic part. 19.8 25.2 22.7 29.5 20.0 --- Inorganic particles 21.5 23.2 20.3 21.6 30.2 --- SM-NOR2 Algal particles 14.4 18.8 15.1 15.7 16.7 17.5 Non-algal organic part. 33.2 25.1 18.5 24.6 22.9 14.3 Inorganic particles 24.2 21.6 26.2 30.5 32.9 29.7 GL-CHL Algal particles 38.6 14.3 13.2 19.6 16.1 --- Non-algal organic part. 6.0 27.9 19.6 24.4 19.3 --- Inorganic particles 12.9 23.4 27.9 29.7 26.5 --- GL-WES Algal particles 30.2 17.4 13.9 24.1 18.6 --- Non-algal organic part. 11.1 24.8 23.3 28.5 27.1 --- Inorganic particles 14.0 23.0 21.0 19.2 21.5 --- GL-MID Algal particles 20.6 21.1 18.0 23.2 21.5 13.6 Non-algal organic part. 22.7 18.8 20.8 33.6 26.8 18.6 Inorganic particles 14.4 20.7 21.2 15.6 14.6 20.2
Table 9. Percent contribution to inverse Secchi transparency by dissolved substances, algal particles, non-algal organic particles, and inorganic particles in 2012.
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Attenuation Component Jun 10 Jul 1 Jul 17 Jul 31 Aug 8 Aug 29 Oct 8 SM-DAM Algal particles 10.5 14.2 10.7 53.5 58.2 52.6 10.8 Non-algal organic part. 23.3 27.0 30.5 17.2 21.7 29.8 18.2 Inorganic particles 34.3 20.9 23.8 11.2 7.6 5.6 31.0 SM-MID Algal particles 11.8 13.0 14.1 46.9 48.6 51.6 11.0 Non-algal organic part. 18.7 27.8 26.2 18.0 26.5 29.9 14.8 Inorganic particles 35.1 20.9 25.5 15.1 11.0 <5 27.3 SM-NW1 Algal particles 10.6 14.9 14.0 30.6 52.0 23.8 9.5 Non-algal organic part. 19.5 27.7 31.6 33.2 19.7 51.2 12.8 Inorganic particles 28.8 22.1 17.3 17.0 13.8 <5 23.7 GL-CHL Algal particles 8.9 12.4 14.7 14.5 11.7 6.3 8.3 Non-algal organic part. 20.1 33.8 25.3 36.0 36.5 49.4 45.7 Inorganic particles 26.5 25.5 27.2 20.2 21.6 <5 <5 GL-MID Algal particles 8.3 12.8 13.1 10.7 19.8 12.9 15.2 Non-algal organic part. 26.3 23.3 28.2 37.8 27.9 30.2 23.8 Inorganic particles 18.4 18.4 22.8 20.1 20.6 9.1 7.4
Table 10. Percent contribution to inverse Secchi transparency by dissolved substances, algal particles, non-algal organic particles, and inorganic particles in 2013.
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Attenuation Component Jun 14 Jun 20 Sep 6 Oct 29 SM-DAM Algal particles 6.3 22.1 26.3 22.3 Non-algal organic part. 19.3 17.0 10.9 32.2 Inorganic particles 43.0 22.6 25.7 12.7 SM-MID Algal particles 7.8 11.8 23.4 30.6 Non-algal organic part. 17.0 23.9 13.3 30.8 Inorganic particles 42.3 20.4 29.6 11.8 SM-NW1 Algal particles 8.5 12.9 25.0 32.4 Non-algal organic part. 15.9 23.9 9.8 18.3 Inorganic particles 37.6 19.3 38.3 24.3 GL-CHL Algal particles 8.0 16.6 12.7 23.3 Non-algal organic part. 12.9 17.5 34.6 39.2 Inorganic particles 22.5 15.5 3.6 <5 GL-MID Algal particles 10.4 17.8 13.3 28.1 Non-algal organic part. 11.9 27.1 35.4 13.9 Inorganic particles 25.5 <5 <5 22.3
Table 11. Percent contribution to inverse Secchi transparency by dissolved substances, algal particles, non-algal organic particles, and inorganic particles in 2014.
Figure 37. Ranges and quartiles of % Inverse Secchi transparency by dissolved substances, algal particles, non-algal organic particles, and inorganic particles for Shadow Mountain Reservoir (left) and Grand Lake (right).
Sources and Sinks of Particles
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Particles affecting transparency in Shadow Mountain Reservoir and Grand Lake originate from both native tributaries (North Fork of the Colorado River, East Inlet, North Inlet) and the
Granby Pump Canal. Particles also are produced in situ (phytoplankton, particles derived from aquatic macrophytes), and particles from any of these sources that are deposited in shallow water can be resuspended. Thus, understanding seasonal and interannual variation in transparency in
Shadow Mountain Reservoir and Grand Lake depends on an understanding of the sources of particles affecting transparency. Such an understanding also depends on knowledge of processes that remove particles from the surface waters of the lakes (e.g., export and settling).
Import of Particles by Tributaries
Transport (import) of particles by tributaries was calculated from daily mean flows and measured particle concentrations (Figure 38). Among the three categories of particles (algal, inorganic, non-algal organic), mass transport generally was highest for inorganic particles; transport of particles to Shadow Mountain Reservoir and Grand Lake was much lower for algal particles than for inorganic or non-algal organic particles. Estimates of mass transport extend over the period of June through October for each year. Thus, these estimates omit a portion of snowmelt runoff.
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Figure 38. Transport (import) of particles by tributaries. Transport was calculated from daily mean flows and measured particle concentrations; particle concentrations were interpolated linearly over time between measured values. Mass transport and temporal patterns of mass transport differed among the four tributaries.
Import of particles by North Inlet and East Inlet was highest near peak snowmelt runoff and then fell to very low levels by early August. Particle transport was much higher in 2013 and
2014 than in 2012. In 2012, particle transport during snowmelt runoff was lower for the North
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Fork of the Colorado River than for North Inlet or East Inlet. In 2013 and 2014, particle transport was higher for the North Fork than for either of the other main tributaries. In 2012 and in 2014, there were spikes in particle transport for the North Fork in mid summer, and there was an increase in transport in 2013 for all three tributaries following the storm in September.
During periods of Farr Pumping, rates of particle transport for the Granby Pump Canal were relatively constant, and particle transport often was higher for the Granby Pump Canal than for the native tributaries. However, particle transport by the North Fork of the Colorado River in
June, 2013 and in June, 2014was much higher than the highest rates of transport measured for the Granby Pump Canal.
The Granby Pump Canal was an important source of particles reaching Shadow Mountain
Reservoir, which suggests that particles transported by the Pump Canal could be a major factor affecting transparency in Shadow Mountain Reservoir and Grand Lake. However, the Pump
Canal often is the dominant source of water for Shadow Mountain Reservoir during the summer months after snowmelt runoff, and particle concentrations for the Pump Canal are not as high as the highest concentrations for the North Fork of the Colorado River. In 2012, transparency in
Shadow Mountain Reservoir and Grand Lake increased when pumping began in mid June and decreased when particle transport by the North Fork of the Colorado River rose in mid summer
(Figure 26). Thus, water carried by the Granby Pump Canal sometimes is a source of dilution for more concentrated sources of particles (e.g., the North Fork of the Colorado River at times of high discharge).
The North Fork of the Colorado River is a particularly important source of particles during times of high discharge and especially when discharge is high during the summer months.
Monitoring at multiple stations on the North Fork of the Colorado River and its tributaries in
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2013 showed that particle concentrations can be very high for the upper reaches of the Colorado
River (Figure 39). As for the Grand Lake tributaries and some of the tributaries in the Upper
Colorado River watershed, particle concentrations for the main stem of the North Fork increase when discharge is high, but the increase is much greater for the main stem of the North Fork than for the Grand Lake tributaries. Crater Gulch, which joins the North Fork upstream of CR-PCU, sometimes has particle concentrations over 1000 mg/L (P. Baber and J. McCutchan, unpublished data). Although the watershed area for Crater Gulch is small in comparison with the watershed area for the Colorado River above Shadow Mountain Reservoir, the particles carried by Crater
Gulch appear to explain much of the difference in particle transport between the North Fork and the native tributaries of Grand Lake.
Figure 39. Concentrations of total suspended solids in 2013 and 2014 for stations on the Colorado River (left) and its tributaries (right).
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In addition to measurements of Secchi transparency that were collected as part of this study, Secchi data collected over the same period by the Grand County Water Information
Network (GCWIN) provide a high-frequency record of changes in transparency. In Figure 40,
GCWIN Secchi data for Shadow Mountain Reservoir and Grand Lake are shown with hydrologic records for the North Fork of the Colorado River and the Grand Lake tributaries.
Significant precipitation events are indicated by vertical dashed lines. In 2012, there was an increase in transparency in Shadow Mountain Reservoir after pumping resumed in June and a subsequent decline in transparency throughout July. Storms that caused spikes in discharge, and presumably spikes in particle concentration, in the North Fork of the Colorado River probably contributed to the decline in transparency in July; there were smaller declines in transparency following other storms in 2012 and 2013. In 2014, a spike in discharge near the end of August was followed by a decline in transparency in Shadow Mountain Reservoir, but multiple shifts in the direction of flow between Shadow Mountain Reservoir and Grand Lake made it difficult to separate the effects of pumping from the effects of particles carried by the North Fork of the
Colorado River in 2014. Nonetheless, the correspondence between spikes in discharge in the
North Fork of the Colorado River and declines in transparency in Shadow Mountain Reservoir suggests that particles carried by the North Fork are an important factor affecting transparency during snowmelt runoff and also during the post-runoff months.
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Figure 40. Secchi data collected by GCWIN and tributary flows, 2012 – 2014. Vertical dashed lines indicate significant precipitation events.
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In situ production of particles
In addition to particles transported by tributaries, some particles affecting transparency originate in situ, particularly within Shadow Mountain Reservoir. Particles originating within
Shadow Mountain Reservoir and Grand Lake include phytoplankton, attached algae, organic particles derived from macrophytes, and lake sediments that become resuspended. Comparisons of particle concentrations across sampling stations, especially when water is pumped from
Granby Reservoir to Shadow Mountain Reservoir and the direction of flow is consistently from
Shadow Mountain Reservoir to Grand Lake, provide some information on in situ production of particles (Figure 41). When the director of flow was from Shadow Mountain Reservoir to Grand
Lake, concentrations of particles generally increased between SM-DAM and SM-MID. For the subset of sampling dates with flow from Shadow Mountain Reservoir to Grand Lake, however, differences in particle concentrations among the stations on Shadow Mountain Reservoir were not statistically significant (Tukey Kramer HSD). These small (but not statistically significant) differences among stations reflect the influence of the North Fork of the Colorado River, as well as growth of phytoplankton, in addition to resuspension of particles in shallow areas.
The concentrations of suspended particles in 2012 were generally similar at SM-NW1 and SM-NOR2, but concentrations of inorganic particles tended to be slightly higher at SM-
NOR2 than at SM-NW1. A slight increase in inorganic particles near the connecting channel is consistent with resuspension of sediment; surface sediment near the connecting channel and near the mouth of the North Fork of the Colorado River is dominated by inorganic particles (typically
60-90% by mass). In addition to the routine monitoring data, results of the mapping studies also show evidence of resuspension in shallow areas, including the area near the connecting channel
(Figures 27, 29, 31).
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Figure 41. Box plots of particle concentrations in surface water at stations on Shadow Mountain Reservoir and Grand Lake. Boxes show medians, quartiles, and ranges; dashed lines show grand means.
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Abundance of phytoplankton was lower in 2012 than in some other years, especially in comparison with 2013 during the period when pumping was stopped. The median concentration of chlorophyll a in 2012 was higher at the north end of Shadow Mountain Reservoir than near the dam and was highest at GL-WES (Figure 41). Very little algal biomass was transported to
Shadow Mountain Reservoir or Grand Lake by their tributaries. The increase in phytoplankton abundance as water flowed from the south end of Shadow Mountain Reservoir toward the
Adams Tunnel in Grand Lake resulted from growth of phytoplankton within the lakes. In 2013 and 2014, the direction of flow often was from Grand Lake to Shadow Mountain Reservoir, and chlorophyll a sometimes was higher near the Shadow Mountain dam than near the connecting channel.
Non-algal organic particles are transported via tributaries to Shadow Mountain Reservoir and Grand Lake from their watersheds and also are produced within the lakes. Macrophytes grow in abundance in Shadow Mountain Reservoir, especially at depths from about 1 – 4 m
(Sisneros 2012). Because of Grand Lake’s great depth, very little light reaches the sediment over much of the lake, and growth of macrophytes in Grand Lake is limited to the shallowest areas of the lake. Although macrophytes contribute to suspended non-algal organic particles in Shadow
Mountain Reservoir, the large body of literature on macrophytes and transparency in lakes shows that high biomass of macrophytes usually is associated with high Secchi transparency (e.g., Hilt et al. 2013). The concentration of non-algal organic particles often is about 30% higher at the north (shallow) end of Shadow Mountain Reservoir than near the dam and the mouth of the
Granby Pump Canal (Figure 41); this increase is due partly to particles derived from macrophytes, but resuspension of particles and other processes within Shadow Mountain
Reservoir probably are more important than macrophytes (see following section on stable isotope
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ratios). Particles transported by the North Fork of the Colorado River also account for some of the increase in particle concentrations as water passes through Shadow Mountain Reservoir.
Particles that settle in shallow water may later be resuspended and add to other sources of particles affecting transparency. Resuspension of sediments can occur in shallow areas at the north end of Shadow Mountain Reservoir, in the connecting channel between Shadow Mountain
Reservoir and Grand Lake, and over a small area in Grand Lake near the connecting channel, where depths are less than about 2 m (Figure 42; also Figures 27, 29, 31). Resuspension also can occur near the margins of the lakes and near the mouth of the North Fork of the Colorado River.
High concentrations of inorganic particles at SM-NOR2 and GL-CHL (Figure 41) suggest that resuspension of particles at the north end of Shadow Mountain Reservoir may be important, occasionally. For shallow-water stations, turbidity occasionally was higher near the bottom than in surface water (Figure 43). On most dates, the difference in turbidity between top and bottom was small and turbidity sometimes was higher at the surface than near the bottom, but turbidity was higher at the bottom on October 29, 2012 at SM-NOR2 and on two dates in 2013 at GL-
CHL.
Figure 42. Vertical cross section of Shadow Mountain Reservoir and Grand Lake along a transect extending from near the inlet of the Granby Pump Canal to the Adams Tunnel.
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Figure 43. Turbidity at SM-NOR2 and GL-CHL, at the surface and near the bottom.
The connecting channel between Shadow Mountain Reservoir and Grand Lake is an area of particular interest with respect to resuspension of particles because of the relatively high flow velocities that can occur there during snowmelt runoff and during pumping. Concentrations of particles were nearly identical at SM-NOR2 and GL-CHL when the flow of water was from
Shadow Mountain Reservoir to Grand Lake (Figure 44). An increase in particle concentrations between these two stations is consistent with resuspension of particles near or within the connecting channel, but resuspension of particles in this area does not appear to have a major effect on transparency. Collectively, information on resuspension of particles in shallow areas suggests that resuspension has a significant effect on transparency, but evidence of resuspension is sporadic. Because of its stochastic nature, precise quantification of the resuspension may require further study focused on this process.
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Figure 44. Concentrations of particles (algal, non-algal organic, inorganic) in 2012 at SM-NOR2 and GL-CHL.
Sinks for particles
Particles transported to Shadow Mountain Reservoir and Grand Lake or produced in situ can be removed by two processes: export and settling.
Water bypassing the Shadow Mountain dam transports particles to the Colorado River, and water diverted to the Adams Tunnel removes particles from Grand Lake. Mass transport of particles leaving the two lakes is determined by the hydrologic fluxes and the concentrations of particles near the Shadow Mountain dam and near the Adams Tunnel. Particle concentrations for SM-DAM are used for calculations of export to the Colorado River, and particle 90
concentrations for GL-MID are used for calculations of export to the Adams Tunnel. Particles also move between Shadow Mountain Reservoir and Grand Lake via water carried by the
Shadow Mountain connecting channel (see section on mass balance, below).
Particles denser than water settle at a velocity that is determined by the density, size
(diameter), and shape of the particles. Stokes law can be used to predict settling velocities of spherical particles (Figure 45), although settling rates of particles in turbulent flow deviates somewhat from predictions of Stoke’s law. The water residence time for Shadow Mountain
Reservoir and for the epilimnion of Grand Lake often is shorter than the time required for small particles to settle over a distance of 5 m (i.e., a distance that is characteristic of the depth over much of Shadow Mountain Reservoir and the epilimnion of Grand Lake). Inorganic particles, which are denser than organic particles, settle faster for a given particle size. In addition to size and density, particle shape affects settling rates. Thus, some relatively large particles (e.g.,
Asterionella formosa) settle more slowly than spherical particles of comparable size.
Figure 45. Effects of diameter on settling velocity (left) and specific cross-sectional area (right) for spherical particles. Settling time is relevant to the time that particles remain in suspension, and cross-sectional area is relevant to the optical properties of suspended particles.
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Small particles tend to settle more slowly than large particles and can travel over long distances before settling, and small particles also have a larger cross-sectional area for a given mass of particles (Figure 45). Consequently, small particles can have a disproportionately large effect on transparency in lakes, all else being equal.
Size spectra of particles were determined by the Utermöhl (inverted microscope) method.
Particles were classified into six size classes, approximated as equivalent spherical diameter (i.e., the diameter of a sphere of equal volume): > 640 µm, 160 – 640 µm, 40 – 160 µm, 10 – 40 µm,
2 – 10 µm, and < 2 µm. Except for the smallest particles, which could not be reliably identified, particles also were identified visually, as follows: algal particles, non-algal organic particles, composite particles, and inorganic particles. Composite particles included aggregates of different types of particles, but the sizes of individual particles in the aggregates and the proportions of algal, non-algal organic, and inorganic particles in the aggregates could not be determined.
Microscopic examination revealed a wide range of particles in suspension in the flowing waters and in the lakes, including intact algal cells, diatom frustules (intact frustules and fragments), fragments of vascular plant material (terrestrial vascular plants and aquatic macrophytes), amorphous organic detritus, bacteria, pollen grains, zooplankton, and mineral particles. The largest particles, including identifiable fragments of terrestrial vascular plants and aquatic macrophytes, were very rare. Although medium-sized particles (10 – 40 µm) were visually dominant in many samples, they did not necessarily have the greatest effect on transparency because particles vary greatly in their optical properties (e.g., transparency, size).
For example, phytoplankton cells are relatively transparent and have less effect on transparency than many other organic particles or inorganic particles of comparable size and density.
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Densities of three size classes of particles are shown in Figures 46 – 48. All of the samples were dominated numerically by the smallest particles. Small particles were particularly abundant in the North Fork of the Colorado River during snowmelt runoff and after storms in
2013 and 2014 (Figure 46). Concentrations of larger particles (2 – 40 µm) also increased in the
North Fork of the Colorado River during times of high flow. Like the other flowing stations, particles carried the Granby Pump Canal were dominated by the smallest particles, but the Pump
Canal sometimes carried higher concentrations of particles in the 2 – 40 µm range than the North
Fork of the Colorado River, except at times of high flow. Particles larger than ~40 μm tend to settle rapidly from suspension (Figure 45) and do not usually affect transparency, except near their source (e.g., near the mouth of a tributary or in shallow areas. During the stop-pump period in 2013, the increase in algal biomass in Shadow Mountain Reservoir was dominated by particles of 10 – 40 µm.
Expected settling times for many particles transported by flowing waters was high relative to water residence time for Shadow Mountain Reservoir or for the epilimnion of Grand
Lake (Figures 4, 45, 46-48). Scanning electron microscopy (SEM) showed that particles not visible by light microscopy were numerically abundant in many samples, but it was not feasible to examine all of the samples by SEM. Nonetheless, the abundance of particles < 2 µm generally was higher than is indicated by the counts shown in Figures 46 – 48. It also is apparent that the generally small size of particles in Shadow Mountain Reservoir, Grand Lake, and their water sources is an important factor affecting transparency in the lakes, because particles tend to remain in suspension long enough to be transported considerable distances and because for a given mass of particles, suspended particles tend to have high cross-sectional area (Figure 45).
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Figure 46. Densities of three size classes of particles (10 – 40 µm, 2 – 10 µm, < 2 µm) for samples from flowing waters.
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Figure 47. Densities of three size classes of particles (10 – 40 µm, 2 – 10 µm, < 2 µm) for samples from Shadow Mountain Reservoir.
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Figure 48. Densities of three size classes of particles (10 – 40 µm, 2 – 10 µm, < 2 µm) for samples from Grand Lake.
Mass Balance of Particles
Transport (import) of particles to and export of particles from Shadow Mountain
Reservoir is shown in Figure 49. Transport was calculated from daily mean flows and measured particle concentrations. Particle concentrations were interpolated linearly over time between measured values; thus, uncertainty in estimates of transport depends partly on the temporal frequency for measurements of particle concentrations. At some times, the mass transport of
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particles entering Shadow Mountain Reservoir was nearly balanced by the mass of particles leaving the reservoir. In August and September, 2012, the mass of particles exported to the
Colorado River (dam bypass) and to Grand Lake (connecting channel) exceeded the mass of particles transported to Shadow Mountain Reservoir by the North Fork of the Colorado River and the Granby Pump Canal. During snowmelt runoff in 2013, import and export of particles were nearly balanced; import of inorganic particles exceeded export of inorganic particles, and export of algal particles and non-algal organic particles exceeded import of organic particles.
From August through October in 2013, export of organic particles, including algal particles and non-algal organic particles, greatly exceeded import of organic particles. In 2015, export of particles from Shadow Mountain Reservoir exceeded import at some times during late snowmelt runoff and also during September when water was being pumped from Granby Reservoir.
Figure 50 shows import and export of particles for Grand Lake. Export of algal and non- algal organic particles from Grand Lake exceeded import of organic particles in August, 2012.
From July through October, import was higher than export of inorganic particles in Grand Lake, and import of total suspended solids exceeded export of TSS from early August through October.
2013 was characterized by two periods (snowmelt runoff in June and early July, and the period following the September storm) when import of particles was very high in comparison with export. In 2014, the balance between import and export of particles changed repeatedly in response to natural hydrologic variation over the year and changes in the rate of pumping from
Granby Reservoir to Shadow Mountain Reservoir. Import of particles was greater than export in
August and September, but only during pumping. Relatively high export of particles from Grand lake during June and October probably reflects antecedent conditions (i.e., import of particles during the preceding month).
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Figure 49. Import and export of particles for Shadow Mountain Reservoir. Transport was calculated from daily mean flows and measured particle concentrations; particle concentrations were interpolated linearly over time between measured values.
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Figure 50. Import and export of particles for Grand Lake. Transport was calculated from daily mean flows and measured particle concentrations; particle concentrations were interpolated linearly over time between measured values.
A summary of mass balance calculations for particles entering and leaving Shadow
Mountain Reservoir is shown in Table 12. From June through October, 2012, export of particles
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from Shadow Mountain Reservoir exceeded import for every category of particles (algal, inorganic, non-algal organic). Algal particles had the largest relative difference between export and import, but the absolute difference between export and import was greater for non-algal organic particles and inorganic particles than for algal particles. In 2013 and 2014, export of both algal and non-algal organic particles generally exceeded import. Algal particles and non- algal particles are generated within Shadow Mountain Reservoir. Gain of inorganic particles may represent resuspension of particles that reached Shadow Mountain Reservoir prior to the study period (e.g., early in snowmelt runoff or in previous years). Excess inorganic particles also may include resuspension of diatom frustules (i.e., particles of algal origin but not identified as such).
Import, kg Export, kg Gain, kg Gain, % 2012 Algal particles 40819 129966 89147 218 Inorganic particles 281924 322343 40419 14 Non-algal organic particles 138231 161876 23645 17 All particles (TSS) 460974 614184 153210 33 2013 Algal particles 9851 243053 233202 2367 Inorganic particles 394367 324444 -69923 -18 Non-algal organic particles 83259 272709 189450 228 All particles (TSS) 487477 840206 352730 72 2014 Algal particles 30827 123428 92601 300 Inorganic particles 420548 451921 31373 7 Non-algal organic particles 113210 173316 60106 53 All particles (TSS) 564585 748664 184080 33
Table 12. Mass balance of particles for Shadow Mountain Reservoir (Jun – Oct). Import calculations include particles transported by the North Fork of the Colorado River, the Granby Pump Canal, and the Shadow Mountain connecting channel (when flow is from Grand Lake). Export calculations include the Shadow Mountain dam bypass and particles transported to Grand Lake by the Shadow Mountain connecting channel.
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Table 13 gives a summary of mass transport of particles for Grand Lake. During the summer months in 2012, chlorophyll concentrations in surface water increased gradually from the south end of Shadow Mountain Reservoir to the middle of Grand Lake (Figure 18), which is reflected by a modest difference between export and import of algal particles. Import of inorganic particles to Grand Lake was dominated by particles from Shadow Mountain Reservoir, but settling removed approximately half the mass of inorganic particles reaching Grand Lake.
Import and export of non-algal organic particles were nearly balanced, and the small differences probably were within the margin of error for these calculations. In 2013, import of particles to
Grand Lake was slightly lower than in 2012, even though snowmelt runoff was higher in 2013 than in 2012; this difference reflects greater transfer of water from Shadow Mountain Reservoir to Grand Lake in 2012 compared with 2013. Export of all categories of particles was much lower in 2013 than in 2012. Only ~20% of the mass of particles entering Grand Lake left via the
Adams Tunnel or export to Shadow Mountain Reservoir, due to settling of particles within
Grand Lake. In contrast with 2012 and 2013, export of inorganic particles and total suspended solids over the months of June – October in 2014 was greater than import; this difference between 2014 and the other two years of the study may reflect the import of inorganic particles prior to peak snowmelt runoff.
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Import, kg Export, kg Gain, kg Gain, % 2012 Algal particles 122281 146674 24393 20 Inorganic particles 310027 172676 -137352 -44 Non-algal organic particles 163516 176799 9283 6 All particles (TSS) 595824 492149 -103675 -17 2013 Algal particles 101956 20547 -81409 -80 Inorganic particles 227849 44055 -183794 -81 Non-algal organic particles 196531 44744 -151788 -77 All particles (TSS) 526337 109346 -416991 -79 2014 Algal particles 48830 69137 20307 42 Inorganic particles 84326 109841 25515 30 Non-algal organic particles 63799 66592 2793 4 All particles (TSS) 196955 245570 48615 25
Table 13. Mass balance of particles for Grand Lake (Jun – Oct). Import calculations include particles transported by North Inlet, East Inlet, and the Shadow Mountain connecting channel (from Shadow Mountain Reservoir). Export calculations include the Shadow Mountain connecting channel (when flow is from Grand Lake to Shadow Mountain Reservoir) and the Adams Tunnel.
Stable Isotope Ratios
Ratios of stable isotopes of carbon (12C, 13C) and nitrogen (15N, 14N) vary considerably among environmental samples, and stable isotope ratios sometimes can be used as natural tracers of sources of organic matter. Where sources of organic matter have unique isotope signatures, mixing models can be used to estimate the contribution of different sources to mixtures such as total suspended particles in lakes. Isotope ratios are reported here in standard delta notation (
13C for carbon, 15N for nitrogen), which is the parts-per-thousand difference in ratio between a
13 15 13 sample and a standard (marine carbonate for C, N2 N); higher values of C reflect higher 13C/12C ratios, and higher values of 15N reflect higher ratios of 15N/14N.
Values of 13 15N for potential sources of suspended organic matter are shown in
Figure 51. During 2012 and 2014, pure samples of phytoplankton from Shadow Mountain
Reservoir had isotopic ratios that are typical of phytoplankton in unproductive lakes. Suspended
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algae were concentrated by density-gradient centrifugation to obtain relatively pure samples of phytoplankton for isotopic analyses; for some samples, the centrifugation was not effective in separation of phytoplankton from other particles, but the method worked well for many samples.
During the stop-pump period in 2013, 15N of phytoplankton was similar to 15N in 2012 and
2014, but 13C was higher (near -18‰). Thus, isotopic signatures of phytoplankton show two distinct groupings.
Figure 51. Stable isotope ratios ( 13C, 15N) of potential sources of suspended organic particles. 50% and 90% probability density ellipses are shown for each of the potential sources of suspended organic particles (natural tributaries, Granby Pump Canal, phytoplankton, aquatic macrophytes). Phytoplankton are separated into two groups: bloom conditions during the 2013 stop-pump period (indicated by shading), and other dates.
The increase in 13C of phytoplankton during the 2013 algal bloom was caused partly by changes in the rate of photosynthetic uptake of inorganic carbon, which affected the isotopic discrimination between dissolved inorganic carbon and algal biomass. 13C of organic matter carried by the Granby Pump Canal was similar to 13C of phytoplankton in Shadow Mountain
Reservoir in 2012 and 2013, but 15N was higher (near +4‰). 13C of particles in the North
Fork of the Colorado River and the Grand Lake tributaries was near -25‰, which is typical of C-
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3 terrestrial plants at high elevation in Colorado; 15N of tributary particles varied from about
+6‰ to as low as about 0‰ during snowmelt in 2014. Macrophytes had a relatively wide range of 13C and 15N but were isotopically distinct from other sources. Particles rinsed from macrophytes (not shown) were isotopically similar to macrophytes; these particles were derived primarily from macrophytes, rather than periphytic algae.
With two tracers ( 13C and 15N) and four potential sources of organic particles
(phytoplankton, Granby Pump Canal, tributaries, macrophytes), it is not possible to calculate exactly the proportional contribution of each source in a mixture of particles, such as the samples of suspended particles collected from Shadow Mountain Reservoir and Grand Lake.
Nonetheless, isotope ratios of suspended particles show that phytoplankton, the Granby Pump
Canal, and tributaries are the most important sources of organic particles in the lakes (Figure 52).
None of the samples of suspended particles had isotope signatures similar to macrophytes; macrophytes certainly contribute to the pool of suspended organic particles in the lakes, but the contribution of macrophytes to suspended organic particles is quite small.
Figure 52. Stable isotope ratios ( 13C, 15N) of suspended particles from Shadow Mountain Reservoir and Grand Lake. Ranges of isotope ratios of potential sources are indicated by probability density ellipses (50%, 90%). Ratios for samples from the 2013 algal bloom, during the period with no pumping, are shown in the left panel; ratios for other samples are shown in the right panel.
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Figure 53. Output from the IsoSource model (Phillips and Gregg 2003) for two samples (GL-CHL, SM-DAM) collected on July 31, 2013.
Figure 54. Ranges and quartiles of the mean contribution of each of the four sources of suspended organic particles (phytoplankton, tributaries, Granby Pump Canal, macrophytes) in Shadow Mountain Reservoir and Grand Lake, based on the IsoSource model.
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For a given sample of suspended particles, there is no single solution to the set of equations that describe the mixing of sources shown in Figure 51; it is possible, however, to estimate the range of possible solutions to these equations through modeling. The IsoSource model (Phillips and Gregg 2003) was used to place bounds on the proportional contribution of each of the four sources to samples of suspended organic matter. Mean isotope ratios for each of the sources (Table 14) were used to define end-members in the mixing model, and frequency distributions for the contributions of each source were generated with IsoSource. Output from the IsoSource model is shown for two samples of suspended particles in Figure 53. Generally, the fraction of organic particles derived from phytoplankton was estimated with high precision
(i.e., the range of possible solutions was small), but there was more uncertainty in the fractional contribution of the tributaries and the Granby Pump Canal. The range of means from modeling, for each of the four sources, is shown in Figure 54 for stations on Shadow Mountain Reservoir and Grand Lake. Isotope ratios suggest that suspended organic particles in the lakes were dominated by algal particles and organic particles from the Granby Pump Canal. Within Shadow
Mountain Reservoir, the contribution of phytoplankton to suspended organic particles increased from near the dam to the northern end of the reservoir. Tributaries were relatively more important as a source of organic particles in Grand Lake than in Shadow Mountain Reservoir, but this reflects the generally higher concentrations of algal particles and particles from the
Granby Pump Canal that are present in Shadow Mountain Reservoir, in comparison with Grand
Lake.
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13C ± SD, ‰ 15N ± SD, ‰ Tributaries -25.4 ± 1.03 +3.8 ± 1.78 Granby Pump Canal -30.5 ± 1.36 +3.6 ± 0.28 Phytoplankton (typical) -30.6 ± 1.25 -1.4 ± 0.55 Phytoplankton (2013 bloom) -18.3 ± 0.71 -0.5 ± 0.38 Macrophytes -16.8 ± 3.22 +4.3 ± 1.47
Table 14. Stable isotope ratios (means ± standard deviation) for sources of suspended organic matter in Shadow Mountain Reservoir and Grand Lake.
Results shown in Figure 53 indicate that, aside from algal particles, the Granby Pump
Canal was the most important source of organic particles in suspension; also, the relative contribution of the Pump Canal to suspended organic particles was highest near the Shadow
Mountain dam and lowest in Grand Lake, as would be expected. The relative importance of the native tributaries, as a source of suspended organic matter, showed the opposite pattern, with the higher proportional contributions in Grand Lake than in Shadow Mountain Reservoir. This pattern reflects settling of some of the particles originating from the Granby Pump Canal, as well as lower concentrations of algal particles in Grand Lake. The contribution of macrophytes was highest near the connecting channel between Shadow Mountain Reservoir and Grand Lake, and was generally higher in Grand Lake than in Shadow Mountain Reservoir. Although macrophytes were not an important source of suspended organic particles at any location, their relatively high importance near the connecting channel probably is due to the distribution of macrophytes in
Shadow Mountain Reservoir and the relatively high flow velocities in shallow water near the connecting channel.
It is important to consider the range of variation in potential sources of organic matter
(Figure 51) and the ranges of variation in the results from the IsoSource model (e.g., Figure 53).
Because there is considerable variation in the isotope ratios of the sources, particularly in 13C
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for phytoplankton and Pump Canal particles and in 15N for tributary particles, there is uncertainty in the end members that are assumed for the mixing-model calculations. Also, isotopic separation among the sources affects uncertainty in estimates of the contributions of sources. Because tributary particles and Pump Canal particles were similar in 15N and differed
13C, it is not as easy to differentiate between these two sources as it is to differentiate between phytoplankton and the other sources. The tributaries may have been a more important source of organic particles than is indicated by results of the modeling shown in
Figure 54. Even so, the isotope data show that the contribution of macrophytes to suspend organic particles is quite small.
Conclusions about Transparency
Transparency in Shadow Mountain Reservoir and Grand Lake is determined largely by concentrations of particles in surface water. Generally, non-algal particles had the greatest effect on transparency during the study period, but inorganic particles and algal particles also were important (Figure 35, 37). Chromatic dissolved organic matter also affects transparency but accounts for a relatively small proportion of variance across stations and over time, after accounting for the effects of other variables. Particle concentrations in surface water are controlled by complex relationships between natural hydrologic variation, factors affecting external sources of particles and production of particles within the lakes, and water management.
Four primary factors (native flows, season or time of year, lake depth, Farr pumping) are particularly important and ultimately control most of the variation in particle concentrations and transparency for Shadow Mountain Reservoir and Grand Lake. The mechanisms that link these
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three primary factors to particle concentrations and their implications for management alternatives are explained below.
Primary Controlling Factors
Particle concentrations in the native tributaries increase with discharge, and in many years, the largest flux of particles from the native tributaries to Shadow Mountain Reservoir and
Grand Lake occurs during snowmelt runoff. The flux of particles transported by North Inlet and
East Inlet during the runoff season can be sufficient in some years to reduce Secchi transparency in Grand Lake to less than 4 m, even without particles from Shadow Mountain Reservoir or from other sources. Settling and flushing gradually remove particles transported to Grand Lake during runoff. Thus, in the absence of other sources of particles, the seasonal pattern of transparency in
Grand Lake probably would be similar to the typical pattern for Dillon Reservoir and many other lakes and reservoirs at similar elevation in Colorado (i.e., minimum transparency in spring or early summer, followed by increasing transparency through summer and fall; Figure 55).
Figure 55. Seasonal variation in Secchi transparency for the Dillon Reservoir Index station, 1981 – 2012. Secchi measurements for 1995 (a wet year) and 2002 (dry year) are highlighted.
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Among the native tributaries, the North Fork of the Colorado River is a particularly potent source of particles, especially during snowmelt runoff and at other times when discharge is high. The increase in particle concentrations with discharge is steeper for the North Fork of the Colorado River than for East Inlet or North Inlet. In 2012, weak flows in the North Fork of the Colorado River during runoff were followed by spikes in flow in July and early August
(Figure 2), and these spikes coincided with high particle transport by the North Fork of the
Colorado River (Figure 20) and low transparency in both Shadow Mountain Reservoir and
Grand Lake (Figures 26, 40). Particle concentrations in the North Fork of the Colorado River also were high in 2013 and 2014 during snowmelt runoff and after a storm in September, 2013
(Figure 19). Particle concentrations sometimes were higher near Shadow Mountain Reservoir than at locations farther upstream on the North Fork of the Colorado River, and land use in the lower Kawuneechee Valley accounts for some of the difference in particle yield between the
North Fork of the Colorado River and the Grand Lake tributaries. However, particle concentrations can be very high at stations far upstream of Shadow Mountain Reservoir (e.g.,
CR-PCU) as a result of particles originating within the Crater Gulch watershed and possibly from other, unidentified areas. Even though the Crater Gulch watershed is small in comparison with the watershed of the Colorado River above Shadow Mountain Reservoir, Crater Gulch accounts for most of the difference in particle transport between the North Fork of the Colorado
River and the Grand Lake tributaries. Particles from the Crater Gulch watershed and other source areas within the North Fork watershed also may affect transparency in Shadow Mountain
Reservoir and Grand Lake indirectly because some water from the North Fork passes to Granby
Reservoir and then is pumped back to Shadow Mountain Reservoir (see following section on
Farr pumping).
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The seasonal timing of runoff in the North Fork of the Colorado River is an important factor controlling particle concentrations in Shadow Mountain Reservoir, but seasonality also is important because the relationship between discharge and particle concentration varies substantially with time of year. At times when snow cover remains at the highest elevations, including parts of the Crater Gulch watershed, some of the most important source areas for particles within the Colorado River watershed are relatively protected from erosional processes.
When precipitation falls directly on areas of bare soil, however, particle concentrations in the
North Fork of the Colorado River can increase dramatically. Thus, for a given discharge, particle yield for the upper Colorado River tends to be greater in August and September than in
May or June.
In addition to erosional processes within the Colorado River watershed, time of year affects the potential for phytoplankton growth. Concentrations of suspended chlorophyll a often are low enough (< 10 µg/L) in Shadow Mountain Reservoir that algal particles have only a modest effect on transparency. However, concentrations of chlorophyll a during the warmest part of the year sometimes are sufficiently high (>30 µg/L) to greatly reduce transparency. Such high abundance of phytoplankton occurs only when water residence time is high, but the combination of warm temperatures and high water residence time is necessary for development of high phytoplankton biomass. Low water residence time prevents development of high phytoplankton biomass because the loss of cells through flushing counterbalances growth of cells, but growth rates of algae also are suppressed by low temperatures. Growth rates of phytoplankton can be suppressed by low water residence time during any season. However, the highest growth rates for phytoplankton occur only during the warm months, when water residence time also is high.
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Lake depth varies greatly between Grand Lake and Shadow Mountain Reservoir (Table
1, Figure 42), and depth is one of the most important factors affecting transparency in lakes generally. Shallow lakes tend to be more productive than deep lakes (Figure 56) because nutrients are more easily transferred from sediments to surface water in shallow lakes. Also, resuspension of sediments due to wind mixing or other causes can affect transparency in shallow lakes. Even without consideration of the particles transported by the North Fork of the Colorado
River, Shadow Mountain Reservoir would be expected to have lower transparency than a deep lake such as Grand Lake. Abundance of macrophytes is another difference between some shallow lakes and deep lakes, and the role of macrophytes on transparency in Shadow Mountain
Reservoir and Grand Lake has been an important question. High abundance of macrophytes usually is associated with clear water because macrophytes stabilize sediments and reduce resuspension (e.g., Horppila and Nurminen 2005). Although macrophytes contribute to suspended organic suspended particles, stable isotope analyses show that macrophytes are not an important source of suspended particles in the lakes (Figures 51 – 54). In fact, the presence of rooted macrophytes in Shadow Mountain Reservoir probably increases rather than decreases transparency in the lakes.
Pumped flow from Granby Reservoir to Shadow Mountain Reservoir (Farr pumping) causes frequent transfer of water from Shadow Mountain Reservoir to Grand Lake, and Grand
Lake often functions as a lake with one small, deep basin and another basin that is larger and much shallower. Thus, the expected transparency for Grand Lake under current operations of the
C-BT Project is different from the expectations for other lakes of similar depth. When water from Shadow Mountain Reservoir enters Grand Lake, particles originating within Shadow
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Figure 56. Relationship between maximum phytoplankton biomass (as chlorophyll a) and maximum depth for world lakes. Data are from the World Lake Database (International Lakes Environment Committee Foundation) and J. McCutchan (unpublished data).
Mountain Reservoir and also particles carried by the North Fork of the Colorado River and the
Granby Pump Canal affect transparency in Grand Lake. During the summer and fall months
(i.e., after snowmelt runoff), particle concentrations in the Granby Pump Canal usually are higher than concentrations in the native tributaries, but not as high as the highest concentrations observed in the North Fork of the Colorado River (Figures 20, 27). However, particle concentrations carried by the North Fork of the Colorado River at times of high flow can be much higher than concentrations of particles in Grand Lake’s two main tributaries. Furthermore, turbulence associated with high rates of pumping destabilizes the water column in Shadow
Mountain Reservoir and may contribute to resuspension of particles in Shadow Mountain
Reservoir and shallow areas of Grand Lake (e.g., near the connecting channel).
The four primary factors affecting particle concentrations and their underlying mechanisms are summarized in Table 15.
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Factor Mechanism Native flows Particle conc. for tributaries increase with discharge; WRT decreases Season (time of year) Temp. affects algal growth, mixed depth; snow cover affects particle yield Lake depth Resuspension of particles; internal loading of nutrients Farr pumping Pumping changes water source for SMR, GL; affects WRT, mixed depth
Table 15. Primary factors affecting particle concentrations and transparency in Shadow Mountain Reservoir and Grand Lake. WRT = water residence time; SMR = Shadow Mountain Reservoir; GL = Grand Lake.
Particles account for nearly all of the variance in transparency in Grand Lake and Shadow
Mountain Reservoir. A multivariate approach (e.g., Tables 7 – 8) is appropriate if the highest precision is required for prediction of transparency, but TSS provides a good basis for prediction of Secchi transparency (Figure 57). Because particle concentrations in the lakes are determined largely by the four primary factors discussed above, it is possible to estimate particle concentrations in each lake for a given set of conditions (i.e., native flows, time of year, Farr pumping; Table 16). Then, transparency in each lake can be predicted from the characteristic particle concentrations for each set of conditions. This simplistic approach to modeling does not consider all of the factors affecting transparency in the lakes. Nonetheless, this approach provides a means to estimate characteristic particle concentrations for the lakes and corresponding values of Secchi transparency over a range of values for each of the three primary factors. The purpose here is to provide a reasonable set of expectations for transparency over a range of conditions, as a means to demonstrate the feasibility of a framework for evaluation of different management alternatives.
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Figure 57. Secchi transparency (with view scope) versus total suspended solids, 2012 - 2014.
Factors Characteristic particle Characteristic Secchi Native Season Farr concentration, mg/L transparency, m flows pumping SMR GL SMR GL High Warm On 5 3 1.5 2 Low Warm On 2.5 2.5 2.5 2.5 High Cool On 3 2 2 3 Low Cool On 2.5 2.5 2.5 2.5 High Warm Off 5 2 2 3 Low Warm Off 8 1 1 4 High Cool Off 3 2 2 3 Low Cool Off 3 <1 2 5 - 8
Table 15. Characteristic particle concentrations and Secchi transparency in Shadow Mountain Reservoir and Grand Lake for different combinations of factors under current operations of the C-BT system. Secchi transparency is estimated here from TSS, according to the relationship given in Figure 57. Shading indicates sets of conditions with Secchi transparency outside the range of ~2 – 4 m that is typical under present operations of the C-BT system.
The seasonal patterns of discharge and particle concentrations in the native tributaries affect seasonality of transparency in Shadow Mountain Reservoir and Grand Lake, but interannual variability in snowmelt runoff is substantial. Also, the effects of summer storms and the timing of Farr pumping are determined by stochastic processes. Under current operations for the C-BT system, Secchi transparency in Grand Lake and Shadow Mountain Reservoir can be
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predicted for a given set of conditions, through use of a simple approach as demonstrated here or through more sophisticated modeling if greater precision is required. Because of the stochastic nature of the primary factors affecting particle concentrations in the lakes, there is considerable variation in transparency for the lakes for any given time of year, and precise predictions of transparency are possible only over short periods of time or for very precisely defined sets of conditions.
Implications for Management Alternatives
Understanding the factors affecting particle concentrations and their links to transparency will be important for development of management alternatives to improve Secchi transparency in
Grand Lake and Shadow Mountain Reservoir. For example, conditions that lead to particle concentrations greater than ~2.5 mg/L as TSS are not consistent with Secchi transparency ≥ 4 m
(Figure 57). Some possible management alternatives are summarized in Table 17.
Hypothetically, diversion of the North Fork of the Colorado River at times of high flow or construction of sedimentation basins above Shadow Mountain Reservoir could mitigate the effects of erosional processes in the Crater Gulch watershed. Because the thin mixed layer in
Grand Lake during the warm months concentrates particles near the surface, alteration of the mixed depth in Grand Lake could improve Secchi transparency. Also, because the highest rates of phytoplankton growth occur during summer, nutrient control could result in greater transparency during summer. Possible management alternatives related to Farr pumping include alterations to the C-BT system or its operations.
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Factor Management alternatives Native flows Diversion of Colo. R. at high flow (e.g., via Redtop); sedimentation basins Lake depth Control sources of suspended particles in SMR; deepen Shadow Mtn. Res. Season (time of year) Alteration of GL mixed depth; nutrient control of phytoplankton growth Farr pumping Major alteration of C-BT; operational control; line pump canal
Table 17. Some management alternatives corresponding to primary factors affecting particles and transparency.
The management alternatives listed above are subject to various constraints and limitations
(Table 18). While diversion of the North Fork of the Colorado River hypothetically could improve transparency in Shadow Mountain Reservoir and Grand Lake at some times, such diversions probably would conflict with service of water rights, and diversion of the highest flows would present significant engineering challenges. To be effective in removal of small particles, such as particles carried by the upper Colorado River, sedimentation basins would have to be large. Maintenance costs for effective sedimentation basins would be high and suitable locations for large basins may not exist. The mixed depth of Grand Lake could be altered by changing the inlet from Shadow Mountain Reservoir or through aeration, but both of these options might have some undesirable consequences. Alteration of the connecting channel could interfere with boat traffic between the lakes, and bubble plumes might be objectionable for aesthetic reasons. Nutrient control of algae may have only limited effects on transparency because nutrient concentrations are not particularly high and further reductions would be difficult; also, some of the sources of nutrients are natural and could not easily be controlled.
Construction costs and effects on power generation are a concern with many possible alternatives involving major alteration of the C-BT system. The potential for operational control is rather limited, although additional storage capacity within the C-BT system or other major alterations could increase the potential for operational control.
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Alternative Comments Diversion of Colo. R. Possible incompatibility with water rights; diversion at high flow difficult Sedimentation basins Limited space for large basins; maintenance (periodic sediment removal) Deepen SMR Costly; slight increase in depth would have limited effect on transparency Alter GL mixed depth Effects on boating (modified inlet); maintenance, aesthetics (aeration) Nutrient control of algae Nutrient conc. are not particularly high; reductions would be difficult Major C-BT alteration Multiple constraints (e.g., construction costs, effects on power generation) Operational control Potential for control dependent on antecedent conditions, storage capacity, etc. Line pump canal Construction costs; benefits not established
Table 18. Comments on management alternatives.
Some of the possible management alternatives that could increase Secchi transparency in
Grand Lake may not be feasible, and some would have unwanted consequences, such as negative effects on water quality in Shadow Mountain Reservoir or elsewhere in the C-BT system.
Results of the 2012 – 2014 particle study will support modeling and will guide future work to better understand factors affecting transparency in Grand Lake. This work also will support analyses to select management alternatives with the best potential for control of transparency in
Grand Lake, under present and future operations, while minimizing unwanted side-effects on the
C-BT system and its operations.
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Appendix I.
Sampling locations and dates
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Sampling date Sampling event 2012 June 1 Mapping June 14 Routine June 20 Routine July 10 Routine August 2 Mapping August 7 Routine September 6 Routine October 29 Abbreviated 2013 June 10 Abbreviated June 26 Watershed July 1 Routine July 17 Combined July 31 Routine August 8 Combined August 19 Watershed August 29 Routine September 10 Watershed October 8 Routine 2014 May 20 Watershed June 10 Combined July 14 Watershed July 29 Mapping July 30 Combined August 25 Routine September 8 Combined
Table I.1. Sampling dates for the 2012-2014 Particle Study.
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Station ID Station name Latitude Longitude Flowing-waters EI-GLU East Inlet near Grand Lake 40.2369 -105.8010 NI-GLU North Inlet near Grand Lake 40.2507 -105.8148 CR-SMU Colorado River at Hwy 34 40.2189 -105.8577 GR-PUMP Granby Pump Canal near SMR 40.2068 -105.8495 GR-PUMP2 Granby Pump Canal upstream 40.1914 -105.8660 CR-NFG Colorado River at North Fork gage 40.3264 -105.8569 CR-SVD Colorado R. downstream of Sun Valley Ranch 40.2771 -105.8504 CR-RTU Colorado R. upstream of Redtop Ditch 40.2523 -105.8685 CR-RTD Colorado R. downstream of Redtop Ditch 40.2488 -105.8672 CR-PCU Colorado R. upstream of Phantom Creek 40.4015 -105.8500 OC-TRU Onahu Creek upstream of Trail Ridge Road 40.3184 -105.8455 BC-TRU Beaver Creek upstream of Trail Ridge Road 40.3949 -105.8446 TC-TRU Timber Creek upstream of Trail Ridge Road 40.3808 -105.8495 TC-NTU Tonahutu Creek upstream of North Inlet Trail 40.2567 -105.8156 Lake stations GL-MID Grand Lake, middle 40.2435 -105.8133 GL-WEST Grand Lake, west 40.2419 -105.8215 GL-CHL Grand Lake near SMCC 40.2447 -105.8258 SM-NOR2 Shadow Mountain Reservoir, north 40.2479 -105.8369 SM-NW1 Shadow Mountain Reservoir, center 40.2370 -105.8418 SM-MID Shadow Mountain Reservoir, middle 40.2252 -105.8378 SM-DAM Shadow Mountain Reservoir, dam 40.2101 -105.8421
Table I.2. Locations and descriptions of primary sampling stations.
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