The Effects of the 2002 Drought on Low Flows
INTRODUCTION
Because of drought conditions the State has experienced in the last few
years, some streams of the State have had historically low flow levels. The
Division has been asked to investigate what impact these low flow levels might
have on the calculated critical flow conditions used in CDPS permits.
From this investigation two questions will be answered. How exceptional
were the 2002 flow levels in relation to the historical flow record? And secondly,
What changes should the Division make to the low flow policy to account for the
low stream flows of 2002?
Balancing these questions requires an understanding of just how rare
these conditions are, and how significant the effects of these flows are on the
calculated critical flows.
The critical flows (i.e. low dilution conditions) are used to establish effluent
limits for CDPS Permits to ensure that water quality standards are not exceeded
at these low flow conditions. The definition of these critical, or low flow,
conditions are included in Regulation No. 31 (The Basic Standards and
Methodologies for Surface Waters, 2001).
31.9
FLOW CONSIDERATIONS
(1)
Low Flow Exceptions
Water quality standards shall apply at all times; provided, that in
developing effluent limitations or other requirements for discharge
permits, the Division shall normally define critical flow conditions
using the following low-flow values: the empirically based 30-day
average low flow with an average 1-in-3-year recurrence interval
(30E3) for chronic (30-day) standards or the empirically based
1-day low flow with an average 1-in-3-year recurrence interval
(1E3) for acute (1-day) standards, or the equivalent
statistically-based flow. For certain substances, such as ammonia,
the low flow exceptions may be based on periodic or seasonal
flows. The length of the periods will be determined on a
case-by-case basis by the Division."
The Division uses historic daily flow records to predict what the low flows
will be in the future. To improve the reliability of the low flow prediction the
Division uses the last 30 years of daily flow records where possible. When the
flow record is less than 30 years, at least ten years of daily flow records are
normally used. Where there are recent significant alterations in the flow regime
of a stream because of flow management it is most appropriate to use only the
last ten years (Paulson & Sanders, 1987).
The Division uses the annual acute (1E3) and annual chronic (30E3) low
flows in calculating WWTF effluent limits. 1E3 means the lowest flow to recur
1
2
every three years, and the 30E3 means the lowest 30-day harmonic mean to
recur every three years. Seasonal (i.e. monthly) low flows are also allowed by
the regulations. The once-in-three year low flow statistic is based on how long it
would take the aquatic community to recover after such low flow conditions
occurred.
To calculate the 1E3 and 30E3 low flows the Division uses EPA DFLOW
software. The DFLOW software is versatile and provides a consistent approach
to the calculation of low flows. The Division has modified this software to allow
for the easy calculation of monthly low flows for use in the Colorado Ammonia
Model (CAM).
ANALYSIS
The Division selected 30 USGS gages for the study (Table 1). The Period
of Record (POR) analyzed included all published data for the listed gages, and
all of the provisional flow data through December 31, 2002. The published data
came from the State Engineers, Office Colorado Decision Support System while
the provisional data came from the USGS NWIS and State Engineers Office
databases. The gages were required to meet certain flow data criteria to be
selected for this analysis:
1)
2)
3)
4)
5)
The gage must be currently active.
The flow record must be from the same location (some gages are moved
over time).
The flow record must be continuous (some records are incomplete
because of gage problems).
The gage must have at least 30 years of daily flow records.
The selection of gages must represent flows across the whole state as
much as possible (Fig. 1).
Table 1 List of USGS Stations in Study
Gage
Number
07086000
07124000
07093700
06711500
09050700
09046600
09085100
09095500
09163500
09070000
07105800
09024000
07083000
09365500
09124500
09260000
09354500
Start of
Record
Location of Gage
Watershed
Elev. (ft) Area (Mi.2)
ARKANSAS RIVER AT GRANITE, CO. 3
1910
8915
427
ARKANSAS RIVER AT LAS ANIMAS, CO. 3
BEAR CREEK AT MOUTH, AT SHERIDAN, CO. 3
1939
1961
1927
3884
6883
5295
14,000
1,480
260
BLUE RIVER BELOW DILLON, CO. 3
1960
8760
335
BLUE RIVER NEAR DILLON, CO. 2
1957
1966
9020
5701
121
6,010
COLORADO RIVER NEAR CO-UT STATE LINE3
1933
1951
4814
4325
8050
17,800
EAGLE RIVER BELOW GYPSUM, CO. 2
1946
6275
944
ARKANSAS RIVER NEAR WELLSVILLE, CO. 3
COLORADO RIVER BELOW GLENWOOD SPGS, CO. 3
COLORADO RIVER NEAR CAMEO, CO. 3
3
1964
5640
495
FRASER RIVER AT WINTER PARK, CO. 2
1910
8906
27.6
HALFMOON CREEK NEAR MALTA, CO.1
1946
9830
23.6
LA PLATA RIVER AT HESPERUS, CO. 3
1917
8105
37.0
FOUNTAIN CREEK AT SECURITY, CO.
2
1937
7828
334
LITTLE SNAKE RIVER NEAR LILY, CO. 3
1921
5685
3,730
LOS PINOS RIVER AT LA BOCA, CO. 3
1951
6144
520
LAKE FORK AT GATEVIEW, CO.
3
Table 1 List of USGS Stations in Study
Gage
Number
09372000
07124200
08251500
09073400
09342500
06714000
06720500
06758500
09110000
09149500
09352900
09304800
09239500
Start of
Record
Location of Gage
Watershed
Elev. (ft) Area (Mi.2)
MCELMO CREEK NEAR CO-UT STATE LINE3
1951
4890
346
PURGATOIRE RIVER AT MADRID, CO. 3
1972
6262
505
1899
7428
7,700
1964
1935
8014
7052
108
298
SOUTH PLATTE RIVER NEAR WELDONA, CO. 3
1895
1926
1952
5158
4999
4308
3,860
4,710
13,200
TAYLOR RIVER AT ALMONT, CO. 3
1910
8011
477
UNCOMPAHGRE RIVER AT DELTA, CO.3
1938
4926
1,120
VALLECITO CREEK NEAR BAYFIELD, CO. 1
1962
7906
72.5
1961
5928
1,020
1910
6695
604
RIO GRANDE NEAR LOBATOS, CO.
3
ROARING FORK RIVER NEAR ASPEN, CO. 2
SAN JUAN RIVER AT PAGOSA SPRINGS, CO. 2
SOUTH PLATTE RIVER AT DENVER, CO. 3
SOUTH PLATTE RIVER AT HENDERSON, CO. 3
WHITE RIVER BELOW MEEKER, CO
2
YAMPA RIVER AT STEAMBOAT SPRINGS, CO. 2
1 - Unregulated USGS Hydrologic Benchmark Stations
2 - Some regulation with a natural hydrograph
3 - Heavily regulated flows
Flows records were analyzed to see how the acute and chronic low flows
varied as the length of flow record was changed. Four record lengths were
analyzed: The full record, ending in December 2002; thirty years (1973 through
2002); ten years (1993-2002) and the latest ten years without 2002 (1992 2001).
RESULTS
The results of the analysis for the full period of record, last 30 years, last
10 years, and latest 10 years without 2002 flows are shown in two tables. Table
2 shows the results for the 1E3 (acute conditions) and Table 3 shows the results
for the 30E3 (chronic conditions).
For each station and each POR, Table 2 and 3 present the low flows in
cfs, and in parenthesis the year(s) in which that flow had actually occurred. For
instance in Table 2, the Arkansas River at Granite has a flow record that starts in
1910. The 1E3 low flow calculated from the entire record is 48 cfs. That flow
actually occurred in 1911, 1912, and 1918. The 1E3 low flow calculated from a
30-year record is 61 cfs and occurred in 1974 and 1976. The 1E3 low flow from
the last ten years of record is 64 cfs and occurred in 2002. The 1E3 low flow
from the latest ten years, that excludes flows from 2002, is 87 cfs and occurred in
1999.
The effects of the 2002 low flow conditions vary from station to station and
also between acute and chronic flows. For some stations, the effects are very
strong resulting in dramatic reductions in the calculated critical low flows. For
other stations, the 2002 flows have little or no effect. Other studies done of 2002
low flows here in Colorado have shown similar results (Saunders, 2003).
4
Table 2 Annual Acute Low Flows
Gage
Number
07086000
07124000
07093700
06711500
09050700
09046600
09085100
09095500
09163500
09070000
07105800
09024000
07083000
09365500
09124500
09260000
09354500
09372000
07124200
08251500
09073400
09342500
06714000
06720500
06758500
09110000
09149500
09352900
09304800
09239500
Location of Gage
ARKANSAS RIVER AT GRANITE, CO. 3
ARKANSAS RIVER AT LAS ANIMAS, CO. 3
ARKANSAS RIVER NEAR WELLSVILLE, CO. 3
BEAR CREEK AT MOUTH, AT SHERIDAN, CO. 3
BLUE RIVER BELOW DILLON, CO. 3
BLUE RIVER NEAR DILLON, CO. 2
COLORADO RIVER BELOW GLENWOOD SPGS, CO. 3
COLORADO RIVER NEAR CAMEO, CO. 3
COLORADO RIVER NEAR CO-UT STATE LINE3
EAGLE RIVER BELOW GYPSUM, CO. 2
FOUNTAIN CREEK AT SECURITY, CO. 3
FRASER RIVER AT WINTER PARK, CO. 2
HALFMOON CREEK NEAR MALTA, CO.1
LA PLATA RIVER AT HESPERUS, CO. 3
LAKE FORK AT GATEVIEW, CO. 2
LITTLE SNAKE RIVER NEAR LILY, CO. 3
LOS PINOS RIVER AT LA BOCA, CO. 3
MCELMO CREEK NEAR CO-UT STATE LINE3
PURGATOIRE RIVER AT MADRID, CO. 3
RIO GRANDE NEAR LOBATOS, CO. 3
ROARING FORK RIVER NEAR ASPEN, CO. 2
SAN JUAN RIVER AT PAGOSA SPRINGS, CO. 2
SOUTH PLATTE RIVER AT DENVER, CO. 3
SOUTH PLATTE RIVER AT HENDERSON, CO. 3
SOUTH PLATTE RIVER NEAR WELDONA, CO. 3
TAYLOR RIVER AT ALMONT, CO. 3
UNCOMPAHGRE RIVER AT DELTA, CO.3
VALLECITO CREEK NEAR BAYFIELD, CO. 1
WHITE RIVER BELOW MEEKER, CO2
YAMPA RIVER AT STEAMBOAT SPRINGS, CO. 2
Flow Record
Started
1910
1939
1961
1927
1960
1957
1966
1933
1951
1946
1964
1910
1946
1917
1937
1921
1951
1951
1972
1899
1964
1935
1895
1926
1952
1910
1938
1962
1961
1910
1E3 for
Full POR
48 (11, 12, 18)
3 (56, 64, 65)
158 (63, 77, 78, 89)
0.5 (54, 55, 63, 67)
12 (63, 84, 85)
17 (93, 95)
908 (81, 01, 02)
856 (34, 35, 36)
1130 (54, 56, 63)
105 (89, 90, 92, 95)
8 (66, 68, 72)
2.5 (12, 56, 85)
1.5 (48)
2.2 (37, 38, 39, 56)
28 (55, 56, 76)
0 (24, 34)
14 (77, 02)
0.2 (51, 56)
3.8 (77, 02)
4.4 (50, 51, 56)
16 (73, 76, 77,94)
15 (50, 53, 56, 02)
23 (25, 51,54, 56, 63)
10 (35, 48, 50, 56)
43 (72, 76, 95, 99, 00)
36 (38, 40, 54)
44 (47, 62, 63)
8.4 (76, 77, 89)
113 (77, 94)
30 (34)
1 - Unregulated USGS Hydrologic Benchmark Stations
2 - Some regulation with a natural hydrograph
3 - Heavily regulated flows
5
1E3 for
Last 30 yrs
61 (75, 76)
4.3 (74)
160 (77, 78, 89)
2.5 (72, 02)
12 (84, 85)
16.9 (93, 95)
882 (81, 01, 02)
1070 (81, 90)
1280 (77)
97 (90, 94, 95)
10 (72, 76, 77)
2.9 (85, 88, 90)
1.8 (76)
2.5 (78, 89, 90)
28 (76)
0 (89, 94, 02)
10 (77, 02)
0.8 (77)
3.8 (77, 02)
6.9 (77, 02)
16 (73, 76, 77, 94)
15 (78, 89, 02)
48 (78, 02)
50 (77, 02)
41 (72, 95, 99)
79 (76, 77,85)
54 (72, 77)
7.5 (76, 02)
95 (77)
42 (77, 02)
1E3 for
Last 10 yrs
64 (02)
7.8 (02)
182 (02)
0.5 (02)
51 (95, 99)
17 (93, 95)
839 (02)
1076 (01, 02)
1308 (02)
72 (02)
41 (92, 93)
3.0 (95)
2.1 (95)
2.8 (02)
29 (95)
0 (94, 02)
8.5 (02)
1.0 (02)
1.8 (02)
4.4 (02)
16 (94)
8.4 (02)
35 (02)
48 (97, 02)
29 (99)
99 (01, 02)
68 (99)
7.3 (02)
80 (02)
23 (02)
1992 - 2001
(w/o 2002 flows)
87 (99)
18 (91)
235 (98)
4.8 (00)
51 (95, 99)
17 (93, 95)
982 (94, 01)
1184 (92, 93)
2015 (99)
103 (92, 94, 95)
41 (92, 93)
3.0 (95)
2.1 (95)
3.9 (00, 01)
29 (95)
0 (94)
22 (90, 96)
3.8 (92)
11 (94)
8.0 (00)
17 (94, 00)
22 (00)
64 (95)
61 (95, 97)
29 (99)
104 (01)
88 (99)
11 (95, 00)
113 (94)
53 (94)
Table 3 Annual Chronic Low Flows
Gage
Number
07086000
07124000
07093700
06711500
09050700
09046600
09085100
09095500
09163500
09070000
07105800
09024000
07083000
09365500
09124500
09260000
09354500
09372000
07124200
08251500
09073400
09342500
06714000
06720500
06758500
09110000
09149500
09352900
09304800
09239500
Location of Gage
ARKANSAS RIVER AT GRANITE, CO. 3
ARKANSAS RIVER AT LAS ANIMAS, CO. 3
ARKANSAS RIVER NEAR WELLSVILLE, CO. 3
BEAR CREEK AT MOUTH, AT SHERIDAN, CO. 3
BLUE RIVER BELOW DILLON, CO. 3
BLUE RIVER NEAR DILLON, CO. 2
COLORADO RIVER BELOW GLENWOOD SPGS, CO. 3
COLORADO RIVER NEAR CAMEO, CO. 3
COLORADO RIVER NEAR CO-UT STATE LINE3
EAGLE RIVER BELOW GYPSUM, CO. 2
FOUNTAIN CREEK AT SECURITY, CO. 3
FRASER RIVER AT WINTER PARK, CO. 2
HALFMOON CREEK NEAR MALTA, CO.1
LA PLATA RIVER AT HESPERUS, CO. 3
LAKE FORK AT GATEVIEW, CO. 2
LITTLE SNAKE RIVER NEAR LILY, CO. 3
LOS PINOS RIVER AT LA BOCA, CO. 3
MCELMO CREEK NEAR CO-UT STATE LINE3
PURGATOIRE RIVER AT MADRID, CO. 3
RIO GRANDE NEAR LOBATOS, CO. 3
ROARING FORK RIVER NEAR ASPEN, CO. 2
SAN JUAN RIVER AT PAGOSA SPRINGS, CO. 2
SOUTH PLATTE RIVER AT DENVER, CO. 3
SOUTH PLATTE RIVER AT HENDERSON, CO. 3
SOUTH PLATTE RIVER NEAR WELDONA, CO. 3
TAYLOR RIVER AT ALMONT, CO. 3
UNCOMPAHGRE RIVER AT DELTA, CO.3
VALLECITO CREEK NEAR BAYFIELD, CO. 1
WHITE RIVER BELOW MEEKER, CO2
YAMPA RIVER AT STEAMBOAT SPRINGS, CO. 2
Flow Record
Started
1910
1939
1961
1927
1960
1957
1966
1933
1951
1946
1964
1910
1946
1917
1937
1921
1951
1951
1972
1899
1964
1935
1895
1926
1952
1910
1938
1962
1961
1910
30E3 for
Full POR
56 (11, 12, 17)
6.3 (54, 56, 64)
215 (62, 63, 64, 77)
1.6 (30, 35, 54)
33 (63, 65, 82, 83, 85)
19 (78, 91)
1065 (77, 81, 01)
1149 (34, 35)
1639 (54, 56, 77)
138 (64, 53, 56, 90, 91, 02)
15 (67, 68, 71)
3.1 (33, 43, 53, 56, 63, 64)
2.2 (48, 75)
3.7 (37, 38, 39, 56, 02)
34 (39, 40, 56, 76)
0 (24, 34)
28 (51, 59, 77, 02)
1.1 (51, 56)
7.9 (72, 73, 02)
9.7 (02, 34, 50, 56)
19 (72, 73, 76, 77)
30 (53, 64, 72, 89, 02)
40 (02, 33, 51, 53, 54, 63)
32 (33, 35, 48, 49)
91 (55, 56, 63, 64, 66, 76)
46 (38, 39, 40)
76 (41, 42, 51)
12 (64, 73, 76, 77)
210 (66, 77, 90)
51 (15, 16, 19, 31, 34)
1 - Unregulated USGS Hydrologic Benchmark Stations
2 - Some regulation with a natural hydrograph
3 - Heavily regulated flows
6
30E3 for
Last 30 yrs
79 (74, 75, 76)
7.1 (74)
214 (76, 77)
4.6 (78, 02)
34 (81, 82, 83, 84, 85)
18 (78, 92, 94)
1053 (77, 81, 02)
1268 (81, 89, 90)
1820 (77)
134 (90, 02)
17 (74, 75, 76, 78)
3.6 (81, 82, 85, 90, 94)
2.4 (73, 75, 76)
3.8 (76, 77, 89, 02)
33 (75, 76)
0.5 (81, 02)
23 (77, 02)
1.6 (77)
7.9 (72, 73, 02)
16 (77)
18 (72, 73, 76)
15 (78, 02)
65 (77, 78, 79, 02)
105 (78, 82, 02)
95 (76, 82, 89, 99)
91 (73)
82 (72, 78, 02)
12 (73, 74, 76)
185 (77, 90)
55 (77, 81, 87, 88)
30E3 for
Last 10 yrs
86 (02)
13 (02)
217 (02)
2.8 (02)
52 (02)
17 (93, 94, 95)
1004 (02)
1270 (01, 02)
1819 (02)
132 (02)
61 (93, 02)
3.4 (94, 95)
2.6 (94, 95)
4.3 (02)
36 (92, 93, 95)
0.2 (02)
19 (02)
1.6 (02)
7.7 (02)
9.8 (02)
21 (02)
21 (02)
62 (02)
82 (02)
95 (95, 99)
103 (02)
81 (02)
12 (01, 02)
166 (02)
56 (02)
1992 - 2001
(w/o 2002 flows)
100 (91, 95, 96)
28 (91)
288 (98)
7.5 (00)
59 (91, 94, 95)
17 (93, 94, 95)
1147 (95)
1405 (92, 93)
2482 (92, 93)
138 (91, 92)
63 (93)
3.4 (94, 95)
2.6 (94, 95)
4.9 (98, 99)
36 (91, 92, 95)
0.6 (92)
36 (96, 99, 00)
12 (96)
11 (94)
22 (96, 00)
22 (94)
44 (00)
85 (94, 95)
132 (95)
95 (95, 99)
113 (97, 98)
135 (92)
14 (99, 00)
232 (94)
71 (94)
2002 Flows in Perspective
Tables 2 and 3 put the low flows of 2002 in perspective. Looking at the entire
record for 1E3 low flows, the flows experienced in 1955 - 1956 have a much greater
effect on the calculated low flows than those of 2002 (11 stations have 1E3 low flows
occurring in 1956, while only 4 have 1E3 low flows occurring in 2002). Looking at the
analysis of the last 30-year period, the drought of 1976 - 1977 has a greater effect than
2002 (17 stations have a 1E3 low flows occurring in 1976 or 1977, while 11 have 1E3
low flows occurring in 2002). The analysis of the most recent ten years shows that of
the last ten years, 2002 is generally but not universally the most frequent low flow year
(21 of the 30 stations have 1E3 low flows in 2002).
In most cases, the acute low flow, calculated from the ten year record (with 2002:
1993 - 2002) is the same or greater than the acute low flow from the full period of
record. Only five stations showed that both the 1E3 and 30E3 low flows that included
the 2002 flow record were lower than those that have occurred historically at these
gages.
There are two important considerations embedded within this analysis of low
flows. One is the protection of designated uses. The other concerns the socioeconomic burden to dischargers. These issues have to be weighed together. It
appears the affect of the 2002 low flows is very site specific, and will have to be judged
on a site specific basis. There is no reason to make a statewide change in the way the
Division handles the calculation of low flows.
Effects on Discharge Permits
Estimating the proper dilution for the effluent from wastewater treatment facilities
(WWTF) is crucial to protecting uses of the receiving water, and preventing an
unreasonable socio-economic burden on the owners of the WWTF. Overestimating the
available dilution could cause impairment to the uses of the receiving water.
Underestimating the available dilution would make the design of the plant greater than
what is needed to protect water quality.
CDPS permits in Colorado can be grouped into three categories. The zero low
flow permits (facilities that discharge to streams that regularly have no flow), permits
with dilution greater than 100:1 (small facilities discharging to large rivers or streams),
and those permits that have some dilution but less than 100:1. The effluent limits for
the zero low flow permits will not be affected. The effluent limits for permits with very
large dilution will also, for the most part, not be affected by the drought because these
permits have limits based on minimum technological abilities and not on dilution.
It is the class of permits with a "medium" amount of dilution that stand some
chance of being affected by drought conditions. Some of these plants will be affected
and others will not be. It is a location specific issue.
1E3 and 30E3 low flows are used in the technical analysis for permits. The
analysis consists of steady state, mass-balance calculations for most pollutants and
modeling for pollutants such as ammonia. The mass-balance equation accounts for the
upstream concentration of a pollutant, critical low flow (minimum dilution), effluent flow
7
and pollutant concentration, and the water quality standard. The mass-balance
equation is expressed as:
M2 
M 3Q3  M 1Q1
Q2
Where:
Q1 = Upstream low flow (1E3 or 30E3)
Q2 = Average daily effluent flow (design capacity)
Q3 = Downstream flow (Q1 + Q2)
M1 = In-stream background pollutant concentrations
M2 = Calculated maximum allowable effluent pollutant concentration
M3 = Maximum allowable in-stream pollutant concentration (water quality
standards).
The calculations of most water quality based effluent limits use the annual or
seasonal acute and chronic low flows. In a general way, since the effluent limits are
proportional to the flows used in the equation, utilizing the above equation can see the
effects of changes in low flows. If in the calculation of new effluent limits you had half of
the previous low flow you would have an effluent limit twice as stringent as the last limit.
Recommendations

WQCC not take any action, flexibility is already allowed by the last line of 31.9(1)
"The length of periods will be determined on a case by case basis by the
Division".

Division should not throw out 2002 flows as appropriate, valuable and historically
consistent flow data should be used where possible.

Continue to watch the situation, 2003 winter flows may also be low.
8
References
The Basic Standards and Methodologies for Surface Water, Regulation 31, CDPHE,
2001
Paulson and Sanders, Evaluation of Design Flow Criteria for Discharge Permits in
Colorado, CSU Low Flow Task, 1987
Saunders, Low Flows and the Drought of 2002 in Colorado, University of Colorado,
2003
9