Willow Creek Reservoir — storage

Storage now, against this reservoir's own record

8.87 Mm³

Stored, September 2026

6th

Percentile among 67 Seps on record

-20.1%

Against the Sep 1991–2020 average

-7.8%

Change over the 12 months to 2026-09

Storage in September 2026 was lower than almost every other Sep on record, ranked against 67 other Seps held for this reservoir.

Storage, 1953–2026

Monthly mean storage

797 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 797 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 797 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
January 1961	8.98
April 1961	10.2
July 1961	9.78
November 1961	9.80
February 1962	9.12
May 1962	9.83
September 1962	12.5
December 1962	9.28
March 1963	6.64
July 1963	10.7
October 1963	11.1
January 1964	8.97
May 1964	9.90
August 1964	12.3
November 1964	10.1
March 1965	10.5
June 1965	12.1
September 1965	10.6
January 1966	9.63
April 1966	10.1
July 1966	9.35
November 1966	9.12
February 1967	10.4
May 1967	10.2
September 1967	11.8
December 1967	9.02
March 1968	10.5
July 1968	9.25
October 1968	12.1
January 1969	9.71
May 1969	10.7
August 1969	11.4
November 1969	10.4
March 1970	11.0
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
October 1973	11.1
January 1974	9.75
April 1974	5.44
August 1974	10.8
November 1974	10.9
February 1975	9.86
June 1975	10.2
September 1975	9.58
December 1975	9.25
April 1976	9.62
July 1976	10.4
October 1976	9.41
February 1977	9.13
May 1977	9.77
August 1977	9.35
December 1977	9.29
March 1978	9.20
June 1978	12.3
October 1978	11.6
January 1979	9.63
April 1979	10.2
August 1979	10.7
November 1979	10.9
February 1980	10.2
June 1980	12.0
September 1980	11.9
July 1986	11.7
November 1986	10.4
July 1987	10.6
November 1987	9.29
August 1988	10.9
November 1988	9.35
February 1989	10.1
June 1989	10.4
September 1989	4.84
December 1989	4.51
April 1990	10.6
July 1990	9.84
October 1990	10.7
February 1991	9.69
May 1991	10.5
August 1991	10.0
December 1991	9.61
March 1992	10.7
June 1992	10.2
October 1992	9.79
January 1993	9.61
April 1993	9.63
August 1993	10.4
November 1993	10.1
February 1994	10.2
June 1994	10.4
September 1994	10.8
December 1994	9.08
April 1995	9.71
July 1995	11.3
October 1995	9.38
February 1996	10.8
May 1996	9.63
August 1996	12.1
December 1996	9.60
March 1997	11.0
June 1997	11.6
October 1997	11.8
January 1998	10.4
April 1998	9.16
August 1998	11.8
November 1998	10.2
February 1999	10.8
June 1999	11.4
September 1999	11.4
December 1999	7.64
April 2000	9.94
July 2000	11.0
October 2000	10.5
February 2001	10.6
May 2001	9.79
August 2001	11.5
January 2002	9.76
April 2002	10.7
July 2002	11.2
November 2002	9.61
February 2003	10.5
May 2003	10.4
September 2003	11.5
December 2003	9.61
March 2004	10.9
July 2004	11.5
October 2004	11.5
January 2005	11.2
May 2005	10.5
August 2005	12.0
November 2005	10.3
March 2006	10.9
June 2006	11.0
September 2006	11.3
January 2007	10.1
April 2007	9.79
July 2007	11.2
November 2007	10.3
February 2008	11.5
May 2008	10.1
September 2008	10.8
December 2008	8.27
March 2009	10.5
July 2009	11.3
October 2009	10.4
January 2010	8.06
April 2010	9.82
August 2010	11.5
November 2010	10.2
February 2011	11.3
June 2011	7.20
September 2011	9.85
December 2011	9.56
April 2012	10.0
July 2012	11.3
October 2012	12.1
February 2013	10.1
May 2013	11.0
August 2013	9.92
December 2013	9.64
March 2014	11.6
June 2014	11.6
October 2014	8.06
January 2015	10.7
April 2015	9.45
August 2015	11.5
November 2015	10.7
February 2016	10.5
June 2016	11.1
September 2016	10.4
December 2016	9.33
April 2017	9.97
July 2017	11.7
October 2017	7.50
February 2018	9.84
May 2018	11.6
August 2018	10.9
December 2018	9.57
March 2019	10.7
June 2019	10.9
October 2019	11.8
January 2020	10.3
April 2020	10.3
August 2020	11.4
November 2020	9.50
February 2021	9.44
June 2021	8.69
September 2021	8.61
December 2021	8.84
April 2022	9.38
July 2022	8.87
October 2022	8.82
February 2023	10.1
May 2023	11.3
August 2023	9.38
December 2023	6.62
March 2024	8.71
June 2024	8.73
October 2024	9.40
January 2025	9.13
April 2025	9.21
August 2025	9.20
November 2025	8.74
February 2026	9.10
June 2026	9.20
September 2026	8.87

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	8.87
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

795 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 795 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 795 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
1956.8333333333333	2,474
1957.0833333333333	2,475
1957.3333333333333	2,475
1957.6666666666667	2,474
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
1962.9166666666667	2,474
1963.1666666666667	2,471
1963.4166666666667	2,475
1963.75	2,476
1964	2,474
1964.25	2,475
1964.5833333333333	2,477
1964.8333333333333	2,475
1965.0833333333333	2,475
1965.4166666666667	2,477
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
1969	2,475
1969.25	2,476
1969.5	2,477
1969.8333333333333	2,475
1970.0833333333333	2,476
1970.3333333333333	2,477
1970.6666666666667	2,477
1970.9166666666667	2,475
1971.1666666666667	2,476
1971.5	2,478
1971.75	2,477
1972	2,476
1972.3333333333333	2,477
1972.5833333333333	2,476
1972.8333333333333	2,476
1973.1666666666667	2,476
1973.4166666666667	2,477
1973.6666666666667	2,477
1974	2,475
1974.25	2,469
1974.5	2,478
1974.75	2,476
1975.0833333333333	2,475
1975.3333333333333	2,475
1975.5833333333333	2,477
1975.9166666666667	2,474
1976.1666666666667	2,475
1976.4166666666667	2,476
1976.75	2,475
1977	2,474
1977.25	2,475
1977.5833333333333	2,475
1977.8333333333333	2,476
1978.0833333333333	2,475
1978.4166666666667	2,477
1978.6666666666667	2,476
1978.9166666666667	2,474
1979.25	2,475
1979.5	2,476
1979.75	2,476
1980.0833333333333	2,475
1980.3333333333333	2,476
1980.5833333333333	2,477
1986.5	2,477
1986.75	2,476
1987	2,476
1987.8333333333333	2,474
1988.0833333333333	2,476
1988.75	2,477
1989.0833333333333	2,475
1989.3333333333333	2,475
1989.5833333333333	2,474
1989.8333333333333	2,468
1990.1666666666667	2,476
1990.4166666666667	2,475
1990.6666666666667	2,476
1991	2,475
1991.25	2,476
1991.5	2,475
1991.8333333333333	2,474
1992.0833333333333	2,476
1992.3333333333333	2,475
1992.6666666666667	2,475
1992.9166666666667	2,474
1993.1666666666667	2,476
1993.5	2,475
1993.75	2,476
1994	2,475
1994.3333333333333	2,476
1994.5833333333333	2,476
1994.8333333333333	2,475
1995.1666666666667	2,475
1995.4166666666667	2,477
1995.6666666666667	2,476
1996	2,476
1996.25	2,474
1996.5	2,477
1996.8333333333333	2,475
1997.0833333333333	2,476
1997.3333333333333	2,475
1997.5833333333333	2,478
1997.9166666666667	2,475
1998.1666666666667	2,476
1998.4166666666667	2,476
1998.75	2,476
1999	2,475
1999.25	2,475
1999.5833333333333	2,477
2000	2,473
2000.25	2,475
2000.5833333333333	2,476
2000.8333333333333	2,475
2001.0833333333333	2,476
2001.4166666666667	2,476
2001.75	2,476
2002	2,475
2002.3333333333333	2,476
2002.5833333333333	2,476
2002.8333333333333	2,475
2003.1666666666667	2,476
2003.4166666666667	2,477
2003.6666666666667	2,477
2004	2,475
2004.25	2,475
2004.5	2,477
2004.8333333333333	2,476
2005.0833333333333	2,477
2005.3333333333333	2,476
2005.6666666666667	2,476
2005.9166666666667	2,475
2006.1666666666667	2,476
2006.4166666666667	2,476
2006.75	2,476
2007	2,475
2007.25	2,475
2007.5833333333333	2,477
2007.8333333333333	2,476
2008.0833333333333	2,477
2008.4166666666667	2,476
2008.6666666666667	2,476
2008.9166666666667	2,473
2009.25	2,476
2009.5	2,476
2009.75	2,476
2010.0833333333333	2,474
2010.3333333333333	2,476
2010.5833333333333	2,477
2010.9166666666667	2,475
2011.1666666666667	2,476
2011.4166666666667	2,472
2011.75	2,474
2012	2,476
2012.25	2,475
2012.5833333333333	2,477
2012.8333333333333	2,475
2013.0833333333333	2,475
2013.4166666666667	2,477
2013.6666666666667	2,476
2013.9166666666667	2,475
2014.25	2,476
2014.5	2,477
2014.75	2,473
2015	2,476
2015.3333333333333	2,474
2015.5833333333333	2,477
2015.8333333333333	2,476
2016.1666666666667	2,476
2016.4166666666667	2,476
2016.6666666666667	2,476
2017	2,475
2017.25	2,475
2017.5	2,477
2017.8333333333333	2,473
2018.0833333333333	2,475
2018.3333333333333	2,477
2018.6666666666667	2,475
2018.9166666666667	2,475
2019.1666666666667	2,476
2019.5	2,477
2019.75	2,477
2020	2,476
2020.3333333333333	2,476
2020.5833333333333	2,477
2020.8333333333333	2,475
2021.1666666666667	2,475
2021.4166666666667	2,475
2021.6666666666667	2,475
2022	2,475
2022.25	2,476
2022.5	2,475
2022.75	2,475
2023.0833333333333	2,476
2023.3333333333333	2,477
2023.5833333333333	2,476
2023.9166666666667	2,472
2024.1666666666667	2,475
2024.4166666666667	2,475
2024.75	2,476
2025	2,475
2025.25	2,475
2025.5833333333333	2,475
2025.8333333333333	2,475
2026.0833333333333	2,475
2026.4166666666667	2,475
2026.6666666666667	2,475

Rate and persistence

-0.06 Mm³

Mean drawdown per month over 12 months

20 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	89.8% 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	2026-10-01
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 2026-10-05, 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

797 monthly observations 1953–2026 Mm³

Spatially aggregatedHigh confidenceSeptember 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 October 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.