Edward Arthur Patterson Lake — storage

Storage now, against this reservoir's own record

7.50 Mm³

Stored, August 2026

43rd

Percentile among 75 Augs on record

-6.8%

Against the Aug 1991–2020 average

-21.2%

Change over the 12 months to 2026-08

Storage in August 2026 was around the middle of the Aug record, ranked against 75 other Augs held for this reservoir.

Storage, 1951–2026

Monthly mean storage

899 monthly values, 1951 to 2026

Monthly means of the publisher's daily record. Volume, not percentage of capacity — see below for why.

What the shaded periods are (0 events)

Not shown on this chart

The 1976 drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
The 2022 drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Yorkshire drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Cantareira water crisis, São Paulo — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Cape Town Day Zero — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
European drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.

View the data behind this chart

240 of 899 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 899 would be longer than this page and unusable with a screen reader.
Period	Edward Arthur Patterson Lake (Mm³)
October 1951	0
February 1952	0
June 1952	7.53
September 1952	5.94
January 1953	5.17
May 1953	7.43
September 1953	8.09
December 1953	6.87
April 1954	9.00
August 1954	5.38
December 1954	4.52
March 1955	5.73
July 1955	8.11
November 1955	5.12
March 1956	5.62
June 1956	6.59
October 1956	5.59
February 1957	5.32
June 1957	8.30
September 1957	7.41
January 1958	6.62
May 1958	7.79
September 1958	6.53
December 1958	5.86
April 1959	8.31
August 1959	5.87
December 1959	6.11
March 1960	7.36
July 1960	7.84
November 1960	5.63
March 1961	6.12
June 1961	5.77
October 1961	4.22
February 1962	3.74
June 1962	8.45
October 1962	6.62
January 1963	6.09
May 1963	8.53
September 1963	6.56
January 1964	5.45
April 1964	5.39
August 1964	7.31
December 1964	6.05
April 1965	8.50
July 1965	8.16
November 1965	6.98
March 1966	8.11
July 1966	8.28
October 1966	7.02
February 1967	6.85
June 1967	8.14
October 1967	5.41
January 1968	5.16
May 1968	5.92
September 1968	5.69
January 1969	5.26
April 1969	8.58
August 1969	7.61
December 1969	6.21
April 1970	8.24
July 1970	7.78
November 1970	6.57
March 1971	8.71
July 1971	7.89
October 1971	8.02
February 1972	8.19
June 1972	8.38
October 1972	7.74
January 1973	8.14
May 1973	8.16
September 1973	5.95
January 1974	6.20
May 1974	7.79
August 1974	6.48
December 1974	5.48
April 1975	7.27
August 1975	7.10
November 1975	6.18
March 1976	8.39
July 1976	7.84
November 1976	5.33
February 1977	5.09
June 1977	8.51
October 1977	8.43
February 1978	8.24
May 1978	8.52
September 1978	7.16
January 1979	7.16
May 1979	8.28
August 1979	6.92
December 1979	6.00
April 1980	7.44
August 1980	4.90
November 1980	4.75
March 1981	4.50
July 1981	4.71
November 1981	7.09
February 1982	8.50
June 1982	12.8
October 1982	12.2
February 1983	13.1
May 1983	12.6
September 1983	8.73
January 1984	7.86
May 1984	12.7
September 1984	10.3
December 1984	9.87
April 1985	10.6
August 1985	8.02
December 1985	7.42
March 1986	13.1
July 1986	13.1
November 1986	12.6
March 1987	12.9
June 1987	11.8
October 1987	11.2
February 1988	10.7
June 1988	10.1
September 1988	6.84
January 1989	5.83
May 1989	12.2
September 1989	9.18
December 1989	8.03
April 1990	7.78
August 1990	7.82
December 1990	5.80
March 1991	5.41
July 1991	5.29
November 1991	3.28
March 1992	3.63
June 1992	3.34
October 1992	2.63
February 1993	2.57
June 1993	6.55
September 1993	8.47
January 1994	7.82
May 1994	12.7
September 1994	10.6
January 1995	11.2
April 1995	12.9
August 1995	12.5
December 1995	10.9
April 1996	10.9
July 1996	7.90
November 1996	7.25
March 1997	11.2
July 1997	10.8
October 1997	8.66
February 1998	8.72
June 1998	10.1
October 1998	9.69
January 1999	10.4
May 1999	10.1
September 1999	7.57
January 2000	7.92
April 2000	8.59
August 2000	6.56
December 2000	6.28
April 2001	10.8
July 2001	10.3
November 2001	9.20
March 2002	9.67
July 2002	9.86
October 2002	7.75
February 2003	8.09
June 2003	9.77
October 2003	7.76
January 2004	7.83
May 2004	10.2
September 2004	7.47
January 2005	6.38
May 2005	8.60
August 2005	4.45
December 2005	2.71
April 2006	5.98
August 2006	7.13
November 2006	6.08
March 2007	6.74
July 2007	8.49
November 2007	5.68
February 2008	5.58
June 2008	5.40
October 2008	3.71
February 2009	2.93
May 2009	10.7
September 2009	3.03
January 2010	1.47
May 2010	10.9
August 2010	9.83
December 2010	9.62
April 2011	11.4
August 2011	10.7
November 2011	9.48
March 2012	10.3
July 2012	9.30
November 2012	7.27
February 2013	8.11
June 2013	10.6
October 2013	10.8
February 2014	10.3
May 2014	11.2
September 2014	10.2
January 2015	8.26
May 2015	10.6
August 2015	5.31
December 2015	5.33
April 2016	6.58
August 2016	5.99
December 2016	6.27
March 2017	10.9
July 2017	8.45
November 2017	7.21
March 2018	8.76
June 2018	10.8
October 2018	9.09
February 2019	10.1
June 2019	10.7
September 2019	10.4
January 2020	10.1
May 2020	8.67
September 2020	7.26
December 2020	5.91
April 2021	6.29
August 2021	5.63
December 2021	5.43
March 2022	5.90
July 2022	10.9
November 2022	8.88
March 2023	9.33
June 2023	10.4
October 2023	9.59
February 2024	8.55
June 2024	9.01
September 2024	7.75
January 2025	7.49
May 2025	10.2
September 2025	7.77
December 2025	7.97
April 2026	9.22
August 2026	7.50

The reservoir's year

Seasonal storage profile

Each calendar month's reference average, against the most recent year

1991–2020 average
Most recent 12 months

Twelve separate averages, one per calendar month. A January average describes Januaries only. Months are plotted 1–12.

View the data behind this chart

Underlying data
Period	1991–2020 average (Mm³)	Most recent 12 months (Mm³)
Jan	7.26	7.86
Feb	7.58	9.73
Mar	8.65	9.77
Apr	9.48	9.22
May	9.48	8.74
Jun	9.35	8.53
Jul	8.79	7.97
Aug	8.05	7.50
Sep	7.60	7.77
Oct	7.19	7.79
Nov	7.09	8.00
Dec	7.11	7.97

Why this page shows a volume and not a percentage

Percentage full is a division, and the denominator is missing

“Percentage full” is the figure a drought is discussed in, and it is stored volume divided by capacity. Capacity is published on several different bases — gross, usable, live, and the figure at a particular spillway or licence condition — and dividing a stored volume by the wrong one produces a percentage confidently wrong by the ratio between them, at exactly the moment the number matters most.

Hydrionis holds storage measurements for this reservoir and does not hold a compatible authoritative capacity figure for it, so percentage full is not calculated. The volume above is what was actually measured. Where you need a percentage, take the publisher’s own capacity on its own stated basis and divide it yourself — the source is linked below, and knowing which basis you used is the whole point.

Water level

Monthly mean water surface elevation

899 monthly values

An elevation above a datum, not a depth of water and not a distance below the crest. Levels at different reservoirs are measured against different datums and must not be read against one another; this line is comparable only with its own history. Elevation and volume are not proportional either — a metre near the bottom of a reservoir holds far less water than a metre near the top, because the surface area grows with height.

What the shaded periods are (0 events)

Not shown on this chart

The 1976 drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
The 2022 drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Yorkshire drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Cantareira water crisis, São Paulo — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
Cape Town Day Zero — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
European drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.

View the data behind this chart

240 of 899 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 899 would be longer than this page and unusable with a screen reader.
Period	Edward Arthur Patterson Lake (m)
1951.75	0
1952.0833333333333	0
1952.4166666666667	736
1952.6666666666667	736
1953	735
1953.3333333333333	736
1953.6666666666667	736
1953.9166666666667	736
1954.25	737
1954.5833333333333	736
1954.9166666666667	735
1955.1666666666667	736
1955.5	736
1955.8333333333333	735
1956.1666666666667	736
1956.4166666666667	736
1956.75	736
1957.0833333333333	735
1957.4166666666667	736
1957.6666666666667	736
1958	736
1958.3333333333333	736
1958.6666666666667	736
1958.9166666666667	736
1959.25	737
1959.5833333333333	736
1959.9166666666667	736
1960.1666666666667	736
1960.5	736
1960.8333333333333	736
1961.1666666666667	736
1961.4166666666667	736
1961.75	735
1962.0833333333333	735
1962.4166666666667	737
1962.75	736
1963	736
1963.3333333333333	737
1963.6666666666667	736
1964	736
1964.25	736
1964.5833333333333	736
1964.9166666666667	736
1965.25	737
1965.5	737
1965.8333333333333	736
1966.1666666666667	736
1966.5	737
1966.75	736
1967.0833333333333	736
1967.4166666666667	737
1967.75	736
1968	735
1968.3333333333333	736
1968.6666666666667	736
1969	735
1969.25	737
1969.5833333333333	736
1969.9166666666667	736
1970.25	737
1970.5	736
1970.8333333333333	736
1971.1666666666667	737
1971.5	736
1971.75	736
1972.0833333333333	737
1972.4166666666667	737
1972.75	736
1973	737
1973.3333333333333	737
1973.6666666666667	736
1974	736
1974.3333333333333	736
1974.5833333333333	736
1974.9166666666667	736
1975.25	736
1975.5833333333333	736
1975.8333333333333	736
1976.1666666666667	737
1976.5	736
1976.8333333333333	735
1977.0833333333333	735
1977.4166666666667	737
1977.75	737
1978.0833333333333	737
1978.3333333333333	737
1978.6666666666667	736
1979	736
1979.3333333333333	737
1979.5833333333333	736
1979.9166666666667	736
1980.25	736
1980.5833333333333	735
1980.8333333333333	735
1981.1666666666667	735
1981.5	735
1981.8333333333333	736
1982.0833333333333	737
1982.4166666666667	738
1982.75	738
1983.0833333333333	738
1983.3333333333333	738
1983.6666666666667	737
1984	736
1984.3333333333333	738
1984.6666666666667	737
1984.9166666666667	737
1985.25	737
1985.5833333333333	736
1985.9166666666667	736
1986.1666666666667	738
1986.5	738
1986.8333333333333	738
1987.1666666666667	738
1987.4166666666667	737
1987.75	737
1988.0833333333333	737
1988.4166666666667	737
1988.6666666666667	736
1989	736
1989.3333333333333	738
1989.6666666666667	737
1989.9166666666667	736
1990.25	736
1990.5833333333333	736
1990.9166666666667	736
1991.1666666666667	736
1991.5	735
1991.8333333333333	734
1992.1666666666667	735
1992.4166666666667	734
1992.75	734
1993.0833333333333	734
1993.4166666666667	736
1993.6666666666667	737
1994	736
1994.3333333333333	738
1994.6666666666667	737
1995	737
1995.25	738
1995.5833333333333	738
1995.9166666666667	738
1996.25	738
1996.5	737
1996.8333333333333	737
1997.1666666666667	738
1997.5	738
1997.75	737
1998.0833333333333	737
1998.4166666666667	737
1998.75	737
1999	738
1999.3333333333333	738
1999.6666666666667	737
2000	737
2000.25	737
2000.5833333333333	737
2000.9166666666667	737
2001.25	738
2001.5	738
2001.8333333333333	737
2002.1666666666667	737
2002.5	737
2002.75	737
2003.0833333333333	737
2003.4166666666667	737
2003.75	737
2004	737
2004.3333333333333	738
2004.6666666666667	737
2005	737
2005.3333333333333	737
2005.5833333333333	736
2005.9166666666667	735
2006.25	736
2006.5833333333333	737
2006.8333333333333	736
2007.1666666666667	737
2007.5	737
2007.8333333333333	736
2008.0833333333333	736
2008.4166666666667	736
2008.75	736
2009.0833333333333	735
2009.3333333333333	738
2009.6666666666667	735
2010	734
2010.3333333333333	738
2010.5833333333333	737
2010.9166666666667	737
2011.25	738
2011.5833333333333	738
2011.8333333333333	737
2012.1666666666667	738
2012.5	737
2012.8333333333333	737
2013.0833333333333	737
2013.4166666666667	738
2013.75	738
2014.0833333333333	738
2014.3333333333333	738
2014.6666666666667	738
2015	737
2015.3333333333333	738
2015.5833333333333	736
2015.9166666666667	736
2016.25	737
2016.5833333333333	736
2016.9166666666667	737
2017.1666666666667	738
2017.5	737
2017.8333333333333	737
2018.1666666666667	737
2018.4166666666667	738
2018.75	737
2019.0833333333333	737
2019.4166666666667	738
2019.6666666666667	738
2020	738
2020.3333333333333	737
2020.6666666666667	737
2020.9166666666667	736
2021.25	737
2021.5833333333333	736
2021.9166666666667	736
2022.1666666666667	736
2022.5	738
2022.8333333333333	737
2023.1666666666667	737
2023.4166666666667	738
2023.75	737
2024.0833333333333	737
2024.4166666666667	737
2024.6666666666667	737
2025	737
2025.3333333333333	738
2025.6666666666667	737
2025.9166666666667	737
2026.25	737
2026.5833333333333	737

Rate and persistence

-0.17 Mm³

Mean drawdown per month over 12 months

0 of 24

Recent months below the 20th percentile for their own month

Rates are computed only across an unbroken run of months, so a rate is never divided by a period in which some of the change was not observed. Percentile counts compare each month with the same calendar month in every other year held.

Location

Edward Arthur Patterson Lake

Gauge location

About this data

Everything below is a property of the data rather than a rating of it. There is deliberately no single confidence score: how much a gap matters depends on what you are about to do with the series, and only you know that.

Source	United States Bureau of Reclamation — Reservoir operations and Colorado River data · US Government work — public domain
How the values came to exist	Measured
Period held	1951–2026
Completeness	98.6% of the periods in that span are held. Gaps are drawn as breaks and never interpolated.
Known gaps	Storage is a managed quantity. A falling line can be a rule curve, a scheduled release or a transfer to another system rather than a shortage of water, and nothing here distinguishes them. No capacity figure on a stated basis is held for this reservoir, so no percentage of capacity is computed.
Continuity	99% of the months between 1951 and 2026 are held. Monthly means of the publisher's own daily record.
Last read from the publisher	2026-09-01
Redistribution	Permitted by the upstream licences
Commercial reuse	Permitted by the upstream licences. No Hydrionis access tier can widen this.
Methodology	How this was produced

Use this data

Machine-readable JSON /api/v1/observations/by-subject/reservoir-usbr-318.json
How to cite this, and who you must credit
How these figures were produced
Request this at a different scale, format or geography

No bulk download is offered here, which is a licence position rather than a product one: at least one contributing publisher does not permit redistribution. The values can be read on the page, cited, and pointed to at source.

Attribution and citation

Suggested citation for Hydrionis

Hydrionis (2026), Edward Arthur Patterson Lake reservoir records, accessed 2026-09-02, hydrionis.com/reservoirs/reservoir-usbr-318/

Cite Hydrionis where you rely on the compilation, the normalisation, the eligibility judgements or a chart. Where you use an unmodified published value, the publisher’s attribution is the one that is required; crediting Hydrionis as well is welcome and is not something Hydrionis can require of you.

Recommended, not required. Hydrionis does not claim rights over unmodified public data it relayed, and doing arithmetic on somebody else’s measurements does not make them Hydrionis’s. Cite the version and the access date: the Edward Arthur Patterson Lake record changes as publishers release and as methods are revised, and a citation without both cannot be checked later. Full guidance is on cite and reuse.

None of the contributing licences requires attribution. Crediting the publishers is still good practice and they are listed above.

Where this comes from

899 monthly observations 1951–2026 Mm³

Spatially aggregatedHigh confidenceAugust 2026

Source: USBR — Reservoir operations and Colorado River data, RISE catalogue item 304. Licence: Open.

Method: Monthly mean of the daily values published by the Bureau of Reclamation in acre-feet, converted to Mm³. Months with fewer than 20 days of record are not published rather than averaged from a partial month.

Stored as a volume and not as a share of capacity: the operator publishes what is in the reservoir but not what it holds when full, so a percentage cannot be computed without inventing the denominator.

Retrieved 1 September 2026.

Monthly mean of the daily values published by the Bureau of Reclamation in acre-feet, converted to Mm³. Months with fewer than 20 days of record are not published rather than averaged from a partial month.