Lake Merwin

{{Use mdy dates|date=February 2025}}

{{Infobox body of water

| name = Lake Merwin

| image = Aerial view of Lake Merwin in Washington (35085174512).jpg

| caption = Aerial view of Lake Merwin

| image_bathymetry =

| caption_bathymetry =

| location = Clark / Cowlitz counties, Washington

| coords = {{coord|45|59|N|122|28|W|type:waterbody_region:US|display=inline,title}}

| type = reservoir

| inflow = Lewis River

| outflow = Lewis River

| catchment = {{convert|731|sqmi|km2|sigfig=3}}

| basin_countries = United States

| length =

| width =

| area = {{convert|3921|acre|km2|sigfig=3}}

| depth =

| max-depth = {{convert|190|ft|abbr=on}}{{cite web|title=Merwin Lake -- Clark County|url=http://www.ecy.wa.gov/programs/eap/lakes/wq/docs/lkmercl1.html|publisher=Department of Ecology, State of Washington|accessdate= 20 December 2014}}

| volume = {{convert|422,400|acre.ft|km3|sigfig=3|lk=in}}{{cite web|title=Reservoirs in the United States| url=http://pubs.usgs.gov/wsp/1838/report.pdf| publisher= United States Geological Survey | accessdate = 20 December 2014}}

| residence_time =

| shore = {{convert|32|mi|abbr=on}}

| elevation = {{convert|72|m|abbr=on|order=flip}}

| islands =

| cities =

| pushpin_map = Washington#USA

| pushpin_label_position =

| pushpin_map_alt = Location of Lake Merwin in Washington, USA.

| pushpin_map_caption =

| website =

| reference = {{GNIS|1523015|Lake Merwin}}[http://npdp.stanford.edu/DamDirectory/DamDetail.jsp?npdp_id=WA00149 Merwin Dam] {{webarchive|url=https://web.archive.org/web/20110716112357/http://npdp.stanford.edu/DamDirectory/DamDetail.jsp?npdp_id=WA00149 |date=2011-07-16 }}, NPDP Dam Directory

}}

Lake Merwin is a reservoir on the Lewis River in the U.S. state of Washington. It lies on the border between Clark County and Cowlitz County. It was created in 1931 with the construction of Merwin Dam.