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Well, you can see the surging water and pouring rain, so the government can order or recommend evacuation to this many people on a very short notice and people will oblige.
Too bad radiation cannot be seen - "invisible snake".
From House of Japan (7/29/2011):
The latest number from NHK Japanese (6:25AM JST 7/30/2011) is 375,000 people (and counting) who have been either ordered or advised to evacuate:Local governments in Niigata and Fukushima prefectures on Friday issued evacuation advisories to thousands of residents due to record rainfall in the two prefectures.
The Japan Meteorological Agency said some areas in Niigata and Fukushima prefectures saw precipitation of about 100 millimeters per hour and warned of continued torrential rain in the two prefectures through Saturday morning.
この豪雨で、新潟県と福島県で、合わせて３７万人余りに避難の指示や勧告が出ています。
Evacuation order and recommendation has been issued to about 320,000 residents in Niigata and Fukushima Prefectures due to the heavy rain.
このうち、新潟県では、三条市で、五十嵐川の堤防が決壊したことを受けて、１万人余りに避難指示が 出されるなど、合わせて１２の市と町のおよそ３６万９０００人に避難の指示や勧告が出されています。また、福島県では、７つの町のおよそ６７００人に避難 の指示や勧告が出されており、新潟と福島の２つの県では、合わせて１２万６０００世帯のおよそ３７万５０００人に避難の指示や勧告が出されています。
In Niigata Prefecture, the levee broke along the Igarashi River in Sanjo City, necessitating the order to evacuate for 10,000 residents. So far, 369,000 residents in 12 cities and towns in Niigata have been ordered or advised to evacuate. In Fukushima Prefecture, 6,700 residents in 7 towns are ordered or advised to evacuate. The total number of affected households in two prefectures is 126,000, or 375,000 residents.
In Fukushima, it's the Aizu region that is affected, which is the west one-third of Fukushima Prefecture.
(h/t Robbie001)