Rex River

{{Use American English|date=February 2025}}

{{Infobox river

| name = Rex River

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| image =

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| map_size = 300

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| pushpin_map = Washington#USA

| pushpin_map_size = 300

| pushpin_map_caption= Location of the mouth of the Rex River in Washington

| subdivision_type1 = Country

| subdivision_name1 = United States

| subdivision_type2 = State

| subdivision_name2 = Washington

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| subdivision_type4 = County

| subdivision_name4 = King

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| discharge1_location= river mile 3.0{{cite web |title= Annual Water-Data Report, USGS 12115500 Rex River near Cedar Falls, WA |publisher= USGS |url= http://wdr.water.usgs.gov/wy2008/pdfs/12115500.2008.pdf |accessdate= 27 August 2009}}

| discharge1_min = {{convert|3.0|cuft/s|m3/s|abbr=on}}

| discharge1_avg = {{convert|100|cuft/s|m3/s|abbr=on}}{{cite web |title= Annual Water-Data Report, USGS 12115500 Rex River near Cedar Falls, WA |publisher= USGS |url= http://wdr.water.usgs.gov/wy2008/pdfs/12115500.2008.pdf |accessdate= 27 August 2009}}

| discharge1_max = {{convert|4200|cuft/s|m3/s|abbr=on}}

| source1 = Cascade Range

| source1_location =

| source1_coordinates= {{coord|47|18|7|N|121|36|9|W|display=inline}}{{GNIS|1525014|Rex River}}

| source1_elevation =

| mouth = Cedar River

| mouth_location = Chester Morse Lake

| mouth_coordinates = {{coord|47|22|43|N|121|41|48|W|display=inline,title}}

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| basin_size = {{convert|13.4|sqmi|abbr=on}}

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The Rex River is a river in the U.S. state of Washington. It originates near the crest of the Cascade Mountains and flows northwest to join the Cedar River in the Chester Morse Lake reservoir. The Cedar River flows to Lake Washington and, ultimately, Puget Sound. Tributaries of the Rex River include Pine Creek, Lindsay Creek, and Boulder Creek.

The Rex River is entirely contained within the Cedar River Municipal Watershed, an area managed by the City of Seattle which provides two-thirds of the water supply for the greater Seattle metropolitan region.{{cite web |title= Cedar River Watershed |publisher= Seattle Public Utilities |url= http://www.seattle.gov/UTIL/About_SPU/Water_System/Water_Sources_&_Treatment/Cedar_River_Watershed/SPU03_001888.asp |accessdate= 27 August 2009}}{{cite web |title= Cedar River Watershed |publisher= USGS |url= http://wa.water.usgs.gov/projects/cedarriver/ |accessdate= 27 August 2009}}