Willow Creek Reservoir — storage

Storage now, against this reservoir's own record

9.21 Mm³

Stored, August 2026

8th

Percentile among 67 Augs on record

-18.7%

Against the Aug 1991–2020 average

+0.1%

Change over the 12 months to 2026-08

Storage in August 2026 was lower than almost every other Aug on record, ranked against 67 other Augs held for this reservoir.

Storage, 1953–2026

Monthly mean storage

796 monthly values, 1953 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 796 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 796 would be longer than this page and unusable with a screen reader.
Period	Willow Creek Reservoir (Mm³)
October 1953	12.7
January 1954	8.59
May 1954	9.63
August 1954	9.32
November 1954	8.48
March 1955	9.43
June 1955	10.9
September 1955	10.0
January 1956	9.06
April 1956	9.37
July 1956	11.2
November 1956	9.14
February 1957	9.67
May 1957	9.41
September 1957	8.63
December 1957	9.51
March 1958	11.2
July 1958	12.7
October 1958	8.47
January 1959	9.38
May 1959	9.43
August 1959	9.96
November 1959	11.0
March 1960	10.8
June 1960	10.2
September 1960	11.5
December 1960	8.70
April 1961	10.2
July 1961	9.78
October 1961	10.6
February 1962	9.12
May 1962	9.83
August 1962	12.6
December 1962	9.28
March 1963	6.64
June 1963	10.2
October 1963	11.1
January 1964	8.97
April 1964	10.1
August 1964	12.3
November 1964	10.1
February 1965	10.1
June 1965	12.1
September 1965	10.6
December 1965	9.45
April 1966	10.1
July 1966	9.35
October 1966	9.29
February 1967	10.4
May 1967	10.2
August 1967	11.9
December 1967	9.02
March 1968	10.5
June 1968	10.8
October 1968	12.1
January 1969	9.71
April 1969	11.0
August 1969	11.4
November 1969	10.4
February 1970	10.4
June 1970	11.8
September 1970	12.2
December 1970	9.49
April 1971	9.00
July 1971	12.8
October 1971	11.8
February 1972	11.0
May 1972	11.4
August 1972	11.0
December 1972	9.40
March 1973	10.9
June 1973	12.1
September 1973	11.6
January 1974	9.75
April 1974	5.44
July 1974	12.5
November 1974	10.9
February 1975	9.86
May 1975	10.2
September 1975	9.58
December 1975	9.25
March 1976	10.2
July 1976	10.4
October 1976	9.41
January 1977	9.05
May 1977	9.77
August 1977	9.35
November 1977	10.6
March 1978	9.20
June 1978	12.3
September 1978	10.8
January 1979	9.63
April 1979	10.2
July 1979	10.6
November 1979	10.9
February 1980	10.2
May 1980	11.3
September 1980	11.9
July 1986	11.7
October 1986	10.7
July 1987	10.6
November 1987	9.29
February 1988	10.2
November 1988	9.35
February 1989	10.1
May 1989	9.88
September 1989	4.84
December 1989	4.51
March 1990	10.4
July 1990	9.84
October 1990	10.7
January 1991	9.44
May 1991	10.5
August 1991	10.0
November 1991	9.26
March 1992	10.7
June 1992	10.2
September 1992	9.91
January 1993	9.61
April 1993	9.63
July 1993	9.98
October 1993	11.1
February 1994	10.2
May 1994	10.5
August 1994	10.5
December 1994	9.08
March 1995	10.0
June 1995	11.8
October 1995	9.38
January 1996	10.2
April 1996	8.69
August 1996	12.1
November 1996	9.72
February 1997	10.9
June 1997	11.6
September 1997	12.3
December 1997	9.66
April 1998	9.16
July 1998	11.9
October 1998	11.2
February 1999	10.8
May 1999	10.3
August 1999	11.5
December 1999	7.64
March 2000	9.72
June 2000	12.0
October 2000	10.5
January 2001	10.1
April 2001	9.69
August 2001	11.5
December 2001	9.35
March 2002	10.4
July 2002	11.2
October 2002	9.98
January 2003	10.2
May 2003	10.4
August 2003	11.6
November 2003	10.2
March 2004	10.9
June 2004	11.2
September 2004	11.6
January 2005	11.2
April 2005	9.83
July 2005	11.3
November 2005	10.3
February 2006	10.4
May 2006	10.0
September 2006	11.3
December 2006	9.62
March 2007	10.8
June 2007	10.3
October 2007	11.2
January 2008	10.9
April 2008	9.66
August 2008	11.4
November 2008	7.45
February 2009	9.69
June 2009	11.4
September 2009	11.9
December 2009	7.61
April 2010	9.82
July 2010	12.0
October 2010	11.1
February 2011	11.3
May 2011	4.02
August 2011	11.0
December 2011	9.56
March 2012	10.9
June 2012	10.6
October 2012	12.1
January 2013	9.77
April 2013	10.2
August 2013	9.92
November 2013	10.6
February 2014	11.0
June 2014	11.6
September 2014	11.3
December 2014	9.80
April 2015	9.45
July 2015	11.4
October 2015	11.5
February 2016	10.5
May 2016	10.6
August 2016	11.4
December 2016	9.33
March 2017	10.7
June 2017	11.8
October 2017	7.50
January 2018	9.35
April 2018	10.1
August 2018	10.9
November 2018	9.78
February 2019	10.3
June 2019	10.9
September 2019	11.7
December 2019	9.74
March 2020	11.4
July 2020	11.0
October 2020	9.73
January 2021	9.17
May 2021	8.94
August 2021	8.69
November 2021	10.1
March 2022	10.2
June 2022	8.77
September 2022	8.84
January 2023	9.49
April 2023	9.38
July 2023	10.8
November 2023	6.73
February 2024	7.77
May 2024	8.17
September 2024	10.1
December 2024	8.49
March 2025	10.1
July 2025	9.42
October 2025	10.4
January 2026	8.79
May 2026	9.19
August 2026	9.21

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	9.95	8.79
Feb	10.4	9.10
Mar	10.7	9.74
Apr	9.76	8.77
May	10.2	9.19
Jun	11.1	9.20
Jul	11.2	9.09
Aug	11.3	9.21
Sep	11.1	9.63
Oct	10.5	10.4
Nov	9.73	8.74
Dec	9.38	8.28

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

794 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 794 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 794 would be longer than this page and unusable with a screen reader.
Period	Willow Creek Reservoir (m)
1953.75	2,478
1954	2,474
1954.3333333333333	2,475
1954.5833333333333	2,475
1954.8333333333333	2,474
1955.1666666666667	2,475
1955.4166666666667	2,476
1955.6666666666667	2,475
1956	2,474
1956.25	2,475
1956.5	2,476
1956.75	2,475
1957.0833333333333	2,475
1957.3333333333333	2,475
1957.5833333333333	2,475
1957.9166666666667	2,475
1958.1666666666667	2,476
1958.4166666666667	2,478
1958.75	2,474
1959	2,475
1959.25	2,476
1959.5833333333333	2,475
1959.8333333333333	2,476
1960.0833333333333	2,475
1960.4166666666667	2,475
1960.6666666666667	2,477
1960.9166666666667	2,474
1961.25	2,475
1961.5	2,475
1961.75	2,476
1962.0833333333333	2,474
1962.3333333333333	2,474
1962.5833333333333	2,478
1962.8333333333333	2,475
1963.1666666666667	2,471
1963.4166666666667	2,475
1963.6666666666667	2,476
1964	2,474
1964.25	2,475
1964.5	2,477
1964.8333333333333	2,475
1965.0833333333333	2,475
1965.3333333333333	2,476
1965.6666666666667	2,476
1965.9166666666667	2,475
1966.1666666666667	2,476
1966.5	2,475
1966.75	2,474
1967	2,475
1967.3333333333333	2,475
1967.5833333333333	2,477
1967.8333333333333	2,475
1968.1666666666667	2,476
1968.4166666666667	2,476
1968.6666666666667	2,477
1968.9166666666667	2,474
1969.25	2,476
1969.5	2,477
1969.75	2,477
1970.0833333333333	2,476
1970.3333333333333	2,477
1970.5833333333333	2,477
1970.9166666666667	2,475
1971.1666666666667	2,476
1971.4166666666667	2,477
1971.75	2,477
1972	2,476
1972.25	2,476
1972.5833333333333	2,476
1972.8333333333333	2,476
1973.0833333333333	2,476
1973.4166666666667	2,477
1973.6666666666667	2,477
1973.9166666666667	2,475
1974.25	2,469
1974.5	2,478
1974.75	2,476
1975	2,475
1975.3333333333333	2,475
1975.5833333333333	2,477
1975.8333333333333	2,475
1976.1666666666667	2,475
1976.4166666666667	2,476
1976.6666666666667	2,475
1977	2,474
1977.25	2,475
1977.5	2,475
1977.8333333333333	2,476
1978.0833333333333	2,475
1978.3333333333333	2,476
1978.6666666666667	2,476
1978.9166666666667	2,474
1979.1666666666667	2,476
1979.5	2,476
1979.75	2,476
1980	2,475
1980.3333333333333	2,476
1980.5833333333333	2,477
1986.4166666666667	2,477
1986.6666666666667	2,476
1987	2,476
1987.75	2,475
1988	2,475
1988.75	2,477
1989	2,475
1989.25	2,476
1989.5833333333333	2,474
1989.8333333333333	2,468
1990.0833333333333	2,475
1990.4166666666667	2,475
1990.6666666666667	2,476
1990.9166666666667	2,474
1991.25	2,476
1991.5	2,475
1991.75	2,475
1992.0833333333333	2,476
1992.3333333333333	2,475
1992.5833333333333	2,475
1992.9166666666667	2,474
1993.1666666666667	2,476
1993.4166666666667	2,476
1993.6666666666667	2,476
1994	2,475
1994.25	2,475
1994.5	2,475
1994.8333333333333	2,475
1995.0833333333333	2,475
1995.3333333333333	2,475
1995.6666666666667	2,476
1995.9166666666667	2,475
1996.1666666666667	2,476
1996.5	2,477
1996.75	2,477
1997	2,476
1997.3333333333333	2,475
1997.5833333333333	2,478
1997.8333333333333	2,474
1998.1666666666667	2,476
1998.4166666666667	2,476
1998.6666666666667	2,477
1999	2,475
1999.25	2,475
1999.5	2,477
1999.75	2,477
2000.25	2,475
2000.5	2,476
2000.75	2,476
2001.0833333333333	2,476
2001.3333333333333	2,475
2001.5833333333333	2,477
2002	2,475
2002.25	2,476
2002.5	2,476
2002.8333333333333	2,475
2003.0833333333333	2,476
2003.3333333333333	2,476
2003.6666666666667	2,477
2003.9166666666667	2,475
2004.1666666666667	2,476
2004.5	2,477
2004.75	2,477
2005	2,476
2005.3333333333333	2,476
2005.5833333333333	2,477
2005.8333333333333	2,476
2006.0833333333333	2,476
2006.4166666666667	2,476
2006.6666666666667	2,477
2006.9166666666667	2,475
2007.25	2,475
2007.5	2,476
2007.75	2,476
2008.0833333333333	2,477
2008.3333333333333	2,475
2008.5833333333333	2,477
2008.9166666666667	2,473
2009.1666666666667	2,476
2009.4166666666667	2,477
2009.75	2,476
2010	2,473
2010.25	2,475
2010.5833333333333	2,477
2010.8333333333333	2,475
2011.0833333333333	2,477
2011.4166666666667	2,472
2011.6666666666667	2,475
2011.9166666666667	2,475
2012.1666666666667	2,476
2012.5	2,477
2012.75	2,477
2013	2,475
2013.3333333333333	2,476
2013.5833333333333	2,475
2013.8333333333333	2,476
2014.1666666666667	2,477
2014.4166666666667	2,477
2014.6666666666667	2,477
2015	2,476
2015.25	2,475
2015.5	2,477
2015.8333333333333	2,476
2016.0833333333333	2,476
2016.3333333333333	2,476
2016.6666666666667	2,476
2016.9166666666667	2,475
2017.1666666666667	2,476
2017.5	2,477
2017.75	2,472
2018	2,475
2018.25	2,475
2018.5833333333333	2,476
2018.8333333333333	2,475
2019.0833333333333	2,476
2019.4166666666667	2,476
2019.6666666666667	2,477
2019.9166666666667	2,475
2020.25	2,476
2020.5	2,476
2020.75	2,475
2021.0833333333333	2,475
2021.3333333333333	2,475
2021.5833333333333	2,475
2021.9166666666667	2,475
2022.1666666666667	2,476
2022.4166666666667	2,475
2022.75	2,475
2023	2,476
2023.25	2,476
2023.5833333333333	2,476
2023.8333333333333	2,472
2024.0833333333333	2,474
2024.3333333333333	2,474
2024.6666666666667	2,476
2024.9166666666667	2,475
2025.1666666666667	2,476
2025.5	2,476
2025.75	2,477
2026	2,475
2026.3333333333333	2,475
2026.5833333333333	2,475

Rate and persistence

+0.00 Mm³

Mean change per month over 12 months

19 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

Willow Creek Reservoir

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	1953–2026
Completeness	89.6% 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	90% of the months between 1953 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-515.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), Willow Creek Reservoir reservoir records, accessed 2026-09-02, hydrionis.com/reservoirs/reservoir-usbr-515/

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 Willow Creek Reservoir 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

796 monthly observations 1953–2026 Mm³

Spatially aggregatedHigh confidenceAugust 2026

Source: USBR — Reservoir operations and Colorado River data, RISE catalogue item 871. 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.