re: Where did all the water that caused the Mystic River flooding come from?

The remnants of a tropical storm and terrain. Alabama has Tornado Alley. Texas has Flash Flood Alley.

Vehicle-Related Flood Fatalities in Texas, 1959–2019

Texas has the highest number of flood fatalities and vehicle-related flood fatalities in the United States. This study provides a detailed analysis of vehicle-related flood fatalities in Texas from 1959 to 2019. The data was compiled from the Storm Data publication maintained by the National Weather Service and includes demographics of the victims, dates, flood types, roadway types, and fatality location. There were 570 vehicle-related flood fatalities during the study period, with almost all fatal accidents resulting in one fatality. These fatalities represent 58% of total flood fatalities. The spatial analysis reveals that most counties with high vehicle-related flood fatalities are clustered in Flash Flood Alley. These counties accounted for over 80% of the fatalities. The annual distribution of these fatalities follows a statistically significant decreasing trend. Monthly distribution of vehicle-related fatalities follows that of rainfall in the Flash Flood Alley, with flash floods causing 61% of all vehicle-related flood fatalities. Night was the time of the day when the most vehicle-related deaths occurred. Males accounted for 63% of the fatalities and the age group of 20–29 was the most affected. The study discusses how the results can be used to increase awareness of flood hazards, used as input into state and regional disaster mitigation plans, and help tailor education and outreach programs….

Floods and Extreme Weather

In Texas, especially in central Texas and West Texas, flash floods are a common and real danger. In hilly terrain, flash floods can strike with little or no advance warning. Be aware of changes in the weather and seek higher ground early. Watch for water rising rapidly in streams and rivers. Watch for unusually hard rain over several hours or steady substantial rain over several days. Never camp on low ground next to streams, bottoms of canyons or deep arroyos since a flash flood can catch you while you're asleep. Distant rain may be channeled into gullies and ravines, turning a quiet stream into a deadly torrent in minutes. Water travels fast - if it's raining upstream you can be flooded even if the sky is clear overhead…

Mapping Flash Flood Severity in the United States…

Figure 2 shows the observed flashiness across the CONUS. At this point, the true spatial distribution of flashiness is limited by the density of the USGS stations with defined flooding thresholds. However, several regions emerge as being prone to flash flooding: 1) the West Coast, 2) Arizona, 3) the Front Range, 4) Flash Flood Alley, 5) the Missouri Valley, and 6) the Appalachians. High flashiness in the West Coast region is restricted to the coastal basins and the upslope region of the Sierra Nevada near Lake Tahoe. Arizona hosts a large number of flashy basins that range from the low deserts in southeastern Arizona all the way up to the Mogollon Rim and the higher-terrain plateau in the northern part of the state. Several flashy basins are apparent just to the east of the Rocky Mountains in the Front Range region. In Texas, several flashy basins are clustered around San Antonio, Austin, and Waco along the Balcones Escarpment in what is locally known as Flash Flood Alley (Flood Safety Education Project 2005). There is a secondary cluster closer to the Gulf Coast near Houston. Moving farther to the northeast, flashiness increases and appears to maximize in the center of Missouri. The Appalachian Mountains in the eastern United States exhibit high values of flashiness from Georgia all the way to Maine.

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I didn't know a Tropical system passed over the area. That explains a lot.

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Technically, a tropical system, Barry only provided the mid level moisture fetches. The real villain was the rare formation of a warm core low on the late evening of July 3, between San Angelo and Midland. This thing exploded just north of San Angelo around 200ish am with 15" rain amounts and points due east in the matter of hours. These systems are very rare and difficult to forecast. Despite that the NWS had watches out by the time the catastrophic rains began.

It was just a tragic event. I dont see anyone at fault.

It came from rain. Like the water in North Carolina Like the many other floods that have occurred.

Until you live out west and see heavy rain on a mountain, desert, or in a canyon you do not understand how fast it runs off.

Everyone is warned to stay out of river beds even if they are dry. Rains 30 miles away an wash you away with no warning.

Rain dibtshit. When it rains in the hills it goes down said hills. That's when math kicks in.
(Didn't explain physics in the first part because it would be a waste of time)...

If you're interested in learning about it. Google Hydraulic time of concentration Guadalupe River Water Basin.

Understanding the principles of time of concentration will help you understand why flooding is different in various watersheds and how topography comes into play.

cloud seeding via HAARP

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I was on that river back in 2007 . 14 people died that day. We had some people tube by us and told us we need to get off the river because there was a big wall of water coming. We kept thinking no way because it hadn’t rained a drop. Sure shite about an hour later the water started to come up. We waded off the rocks and up to land then back up to our rental house. What we watched after that was sheer disbelief. But I’ll say it was nothing compared to what we are watching today. It had rained its arse off miles upstream and not a drop where we were. I remember the military choppers coming in and out and I so vividly remember the RV campground looking like a war zone .

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11 inches over thousands of acres, all funneling into one water body.

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And even with all of that canyon lake is only about 64% full.

An event like this would need to happen two more times to fill it up That's how low it had gotten from the drought.

Ground is really dry. That area is not used to bunch of rain. Also very hilly, so the water follows gravity and surges to the lowest area…. Quickly.

It’s not government weather machines, it’s not climate change. Crazy rainfall happens sometimes.
People think hurricanes are getting worse but the reality is that we are just building more expensive shite where the hurricanes go.